Department of Zoology, University of the Witwatersrand ...7, 8, PI. 11) and the commissura quadrato-cranialis anterior. The ventral muscles of the larval head are two broad trans-verse

The Head of Xenopus laevls.By

Nellie F. Paterson, D.Se., Ph.D.,Department of Zoology, University of the Witwatersrand, Johannesburg.

W i t h P l a t e s 9 t o 16 .

C O N T E N T S . P A G E

I N T R O D U C T I O N 1 6 1

L A T E R A L L I N E S E N S O R Y O R G A N S 163

M U S C U L A T U R E 1 6 5

B L O O D - V E S S E L S . . . . . . . . . 172

T H E C H O N D R O C R A N I U M . . . . . . . . 1 7 5

1. M e t a m o r p h o s i s . . . . . . . . 1 8 3

2 . O l f ac to ry E e g i o n 188

3 . N a s a l Cavi t ies 191

4 . A u d i t o r y E e g i o n . . . . . . . . 193

T H E H Y O B R A N C H I A L S K E L E T O N . . . . . . . 196

T H E C R A N I A L N E R V E S 198

G a n g l i o n P r o - o t i c u m . . . . . . . . 199

N e r v u s T r i g e m i n u s . . . . . . . . 2 0 0

1. R a m u s M a n d i b u l a r i s 2 0 0

2 . R a m u s O p h t h a l m i c u s P r o f u n d u s 2 0 3

N E R V U S F A C I A L I S 2 0 9

1. T r u n c u s S u p r a - o r b i t a l i s 2 1 0

2 . R a m u s H y o m a n d i b u l a r i s . . . . • - . 2 1 1

3 . R a m u s P a l a t i n u s . . . . . . . . 2 1 4

NERVI GLOSSOFHARYNGEtTS AND VAGUS . . . - . 2 1 6

1. Nervus Glossopharyngeus . . . • • • . 2 1 72. Nervus Vagus 220

SUMMARY OF COMPOSITION AND DISTRIBUTION OF N E R V E S . . 226

SUMMARY 227

R E F E R E N C E S 228

INTRODUCTION.

THE Aglossa, comprising only the genera Xenopus, Pipa,Propipa, Hymenochirus, and Pseudohymeno-chirus, are characterized among other things by the absenceof a tongue and by a pectoral girdle that exhibits considerabledeviation from that of typical Anura Phaneroglossa.

NO. 322 M

162 NELLIE F. PATBESON

The Aglossa are usually classified as the lowest of the A n u r a,but as Gadow in his account of the Amphibia in the ' CambridgeNatural History' (1909) indicates, their characteristic featuresare not necessarily primitive ones. A tongue is lacking in themajority of truly aquatic forms, and in the Aglossa the shouldergirdle and other parts of the body are doubtless specialized inresponse to their particular habits. It is therefore not surprisingto find that the Aglossa present some striking morphologicalsimilarities with the aquatic Urodela on the one hand, andwith certain genera of the Phane rog lo s sa on the other,but it is very doubtful if these resemblances are of any conse-quence. Probably they are merely accidental or superficiallikenesses, as for instance the similarities of skeletal structuresindicated in the course of the present study as existing betweenXenopus laev is and the Phaneroglossangenera H e m i s u s ,B r e v i c e p s , and P r o b r e v i c e p s . The resemblances be-tween Xenopus and such Urodela as Siren and P r o -t e u s , especially in regard to the arrangement of the nervoussystem, seem to be more significant, but even this may be dueto adaptation to somewhat similar habitats.

The anatomical peculiarities of X . laevis Daud., the' clawed toad' or ' platanna' of Southern Africa, have for manyyears been favourite subjects of investigation by students ofmorphology, but despite the valuable contributions that haveappeared, there still remain many problematic points.

X. laevis abounds in ponds and dams throughout southernand tropical Africa. Eose (1929) in his book 'Veld and Vlei'introduces the reader to a discussion on Xenopus by statingthat 'the plathander has an even stronger claim than thespringbok to be considered a typically South African animal,for whereas horned antelopes are found in many other countries,clawed toads are found in southern and tropical Africa and inno other part of the world'.

The larvae have been described and figured by Bles (1904) andPeter (1930), and are chiefly remarkable for the extreme trans-parency of the tissues. They may also be recognized by thepresence of a pair of long trailing' tentacles' (tent, fig. 30 a, PL 16),which disappear at metamorphosis, and by the characteristic

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attitude they take up in the water; they usually remain moreor less stationary with the head directed downwards and withthe thin tapering extremity of the tail constantly vibrating andthus enabling them to maintain this position.

The present study of X. l aev is was begun with the inten-tion of investigating the distribution of the cranial nerves, butas it was necessary to make a somewhat intensive study ofother associated cephalic structures and, as certain of theseobservations have proved of interest, they have been includedin the dissertation.

For the purpose of this study a series of larvae in progressivestages of development and measuring from 5 mm. to 60 mm.long were sectioned after having been fixed in aqueous Bouin.In order that comparisons between larval and adult conditionsmight be made possible, and conclusions might be arrived atregarding the changes occurring during the process of meta-morphosis, several transforming specimens as well as some veryyoung frogs were also examined. The latter were sectioned afterhaving been decalcified for several days in 3 per cent, nitricacid as prescribed in the eighth edition of Bolles Lee's 'VadeMeeum', 1921, pp. 252-3. The thickness of all the sections was10/*, and the stains used were Hansen's haematoxylin andeosin. The preparations were found to be very satisfactory fora general study of the various organs and systems in the head,including the cranial nerves.

LATERAL LINE SENSORY ORGANS.

As Xenopus is one of those interesting Anura in whichthe lateral line sensory organs persist throughout life, due nodoubt to their aquatic existence, a comparison of their arrange-ment in larval and adult stages is of some interest. Escher(1925), who gives a diagram of the organs in the adult, failedto observe them in the larva. It is to be admitted, however,that, owing to the light sprinkling of somewhat star-shapedpatches of pigmentation, and more particularly to the remark-able transparency of the head region, the larval sensory organsare rather obscured and are only determined with difficulty.

The ventral organs are easily distinguished as white markings

164 NELLIE F. PATERSON

on the darkly coloured abdominal region in all larval stages.Even in young tadpoles measuring about 15 mm. in length andin which the hind-limbs have not yet been protruded, the organsof the dorsal and median rows (osl., msl, fig. 30 a, PI. 16) aresharply contrasted with the darker body coloration betweenthe fore- and hind-limb buds. On the dorsal and ventral sur-faces of the head in front of this region the organs are indis-tinguishable macroscopically from the general surface of thebody. That they are developed, however, is evident from sectionsof the anterior part of the body. Certain organs (csl., fig. 30 a,PI. 16) are also obvious in these early stages on the ventral finanterior to the cloaca, where they at first appear as a lateralrow of small rounded dots which show a gradual elongationduring subsequent stages of development. In older larvae andin those undergoing metamorphosis these cloacal sensory organsdecrease to a few vertically elongate organs on each side of thefin fold, and with the gradual absorption of the tail they cometo lie even nearer to the cloacal aperture. Thus in the maturefrog they are only observed with difficulty as indistinct ridgeson the ventral fold of skin immediately anterior to the cloacalaperture.

All the sensory organs become more apparent just beforemetamorphosis, when the skin is more deeply and more uni-formly pigmented and the whitish organs show up by contrast.It is then evident that the arrangement in the larva is essentiallysimilar to that of the adult, and that both conform in general tothe plan drawn up by Escher (1925) for U r o d e 1 e Amphibians.The dorsal, median, and ventral rows of trunk sensory organsare strongly developed, the two former systems extending onto the tail (fig. 30 b, PI. 16). The dorsal row may be continuedonly for a short distance caudally in some specimens, but themedian organs are apparent throughout the length of the tailat the dorsal ends of the myomeres.

The cephalic organs are arranged into supra-, infra-, and post-orbital series above, and oral and gular rows below. In addition,as Escher (1925) has indicated in the adult X e n o p u s , thereare hyomandibular organs (hso., fig. 30 b, PI. 16) which arelaterally continuous with the median line on the body, an

HEAD OF XENOPUS 165

accessory row (ace, fig. 30 b, PL 16) which arches dorsally, andseveral small round parietal organs (par., fig. 30 b, PI. 16) situateddorsally midway between the supra-orbitals.

The chief difference between larval and adult arrangementsis found in the orbital grouping. In the larva the supra-orbitals(sorb., fig. 30 b, PI. 16) and post-orbitals (porb., fig. 30 b, PL 16)are quite distinct, but during metamorphosis they graduallyapproach the eye, so that in the adult they are arranged into asingle circumorbital series. Closer inspection reveals that thereis some considerable variation in the arrangement and numberof organs in the several rows, not only in different specimens butalso in the two sides of an individual. In general, however, eachseries of sensory organs is as a whole sufficiently constant toallow of their relative positions being easily determined.

MtTSCULATUBE.

Edgeworth (1929, 1935) has reported upon the cephalicmuscles of the larva of X. f raser i in the course of his well-known dissertations on the muscles of Vertebrates, and theentire musculature of the adult X. laevis has been studiedin detail by Grobbelaar (1924). The present series of prepara-tions of both larval and adult X. laevis have made itpossible to draw conclusions regarding the changes undergoneby the musculature of the head during metamorphosis.

The muscles associated with the chondrocranium up to thetime of metamorphosis are essentially similar to those of X.f r a se r i . The levatores mandibulae anterior and posterior aretwo rather broad muscles arising dorsally on the palatoquadrateand having their insertions on the dorsal surface of the Meckel'scartilage. The levator mandibulae posterior (lev.post, figs. 7, 8,PL 11) lies ventrally to the levator mandibulae anterior (lev.ant.,figs. 6-8, PL 11), than which it is also much shorter, as it onlyextends from the posterior part of the processus muscularis ofthe palatoquadrate, whereas the levator mandibulae anteriororiginates farther back on the subocular bar of the palato-quadrate. As in X . f raser i these two muscles maintaintheir relative positions throughout, and there is no reversal of

166 NELLIE F. PATEBSON

positions such as appears in larval Anura P h a n e r o g l o s s a(Edgeworth, 1924,1935).

In addition to these two masticatory muscles, the larval X.laev is has a third horizontally placed muscle (lev.tent, figs.6-8, PI. 11), which seems to be a portion of the levatormandibulaeanterior, as it arises just laterally to the latter muscle on thelateral margin of the subocular bar of the palatoquadrate. Itsoon separates from the levator mandibulae muscle, and as itpasses forwards to the base of the tentacle it appears dorso-laterally above the mandibular muscle in the deep depressionbetween the pronounced processus muscularis (mus.fr., figs.7, 8, PI. 11) and the commissura quadrato-cranialis anterior.

The ventral muscles of the larval head are two broad trans-verse sheets, the musculi intermandibularis and interhyoideus(int.hy., figs. 6, 7, PI. 11), lying between the Meckel's cartilagesand the cerato-hyalia respectively and each joining its fellowmid-ventrally in an obvious median raphe.

The levator hyoideus is a strongly developed verticallysituated muscle, divisible into two somewhat fan-shaped fasci-culi. The outer, larger fasciculus (lev.hy., figs. 7, 8, PI. 11),termed the orbito-hyoideus by Edgeworth (1935), arises in X.1 a e v i s on the processus muscularis of the palatoquadrate andpasses to a distinct ventro-lateral process of the cerato-hyale.This fasciculus partly overlies the second, rather smaller one(dep.man., figs. 6-8, PI. 11), the quadrato-hyoangularis of X.fraser i (Edgeworth, 1935). In X. l aev is it originates dorso-laterally on the cerato-hyale anterior to its articulation withthe palatoquadrate and spreads out fanwise immediately belowthe processus muscularis. Edgeworth (1935) finds that in X.fraser i it also originates on the quadrate, but in X. l aev i sno distinct evidence of this could be observed. Its fibres, how-ever, lie close to the outer angle of the quadrate just below theprocessus muscularis but seem to have no connexion with it.The muscle passes forwards in front of the insertion of theorbito-hyoideus, below the suspensorial region, and is insertedventro-laterally on the Meckel's cartilage, to which it acts asa depressor muscle.

As in X. f raser i and Pip a, the muscles connected with

HEAD OF XENOPUS 167

the branchial skeleton are simpler in their arrangement thanare those of the Anura P h a n e r o g l o s s a . The dorsalmuscles consist only of the constrictores branchiales, thelevatores arcuum branchialium being unrepresented. The con-strictores arise on parts of the chondrocranium and pass to theventro-lateral region of the cerato-branchialia. In both P ip aand X . f raser i Edgeworth (1935) has recorded four con-strictor muscles, those in P ip a all arising on the auditorycapsule. In X. f raser i the first originates on the ascendingand ventro-lateral processes of the quadrate and the remainingthree on the auditory capsule. In X . laevis the constrictoresbranchiales form an almost continuous lateral sheet of muscle,in which the limits of the four separate muscles can only bemade out with difficulty. The first arises on the subocular barof the palatoquadrate; the second originates on the ventro-lateral process of the palatoquadrate; while the third, which isthe broadest, is located on the crista parotica and commissurabrancbio-cranialis. The fourth is a small muscle situated at theposterior end of the branchial skeleton.

The hypobranchial muscles consist simply of one broadtransversus ventralis springing from the second branchial arch,meeting its fellow in a mid-ventral raphe, and of four separatesubarcuales recti. The latter pass ventrally between the bran-chial arches, the anterior one running from the first arch to thecerato-hyale, as in larvae of Anura Phane rog los sa (Edge-worth, 1919,1935). In addition to these there is another musclewhich does not seem to have been previously recorded. It lieson the inner side of the last branchial arch and forms a broadlongitudinal muscle extending backwards beneath the pharynx.At the junction of the latter with the oesophagus the fibres ofthe two sides become confluent below the oesophagus justanteriorly to the constrictor oesophagi. The muscle continuesposteriorly, attached ventrally to the branchial bar and withits dorsal fibres lying just above the dilatator laryngis. Subse-quently it is attached to the posterior angle of the chondro-cranium close to the origin of the dilatator laryngis. It is alsocontinued obliquely over the posterior part of the branchialskeleton on to the commissura branchialis, immediately above

168 NELLIE F. PATEKSON

the constrictor branchialis IV, with which it appears to fuse.Edgeworth (1935) states that in E a n a the transversus ven-tralis IV is incompletely developed and that in Xenopus itis absent, probably having atrophied at an early stage. Thislongitudinal muscle is somewhat suggestive of the transversusventralis IV of some Pisces and other Amphibia, althoughit is more dorsal in position and does not meet in a median raphe.Its relation with the constrictor oesophagi seems to indicate thepossible origin of the latter from this muscle, and furthermore,the posterior part arching dorsally over the branchial skeleton,may represent a levator arcuum branchialium which has joinedup with the constrictor branchialis IV.

Metamorphosis of the Anura Aglossa is not accompaniedby any such marked changes in the musculature as may occurin other Amphibia. The arrangement of the cephalic musclesin young forms of X. l aev is is, therefore, comparativelysimple and directly comparable with that of the larva. Themain differences between the two conditions are found in thechanged positions of the muscle insertions consequent uponthe backward migration of the suspensorial region which takesplace at metamorphosis.

The levatores mandibulae anterior and posterior of the larvaare two very strong muscles in the young frog, passing from thefronto-parietale round the lateral wall of the auditory capsule tothe lower jaw (lev. man., figs. 18 a, b, PI. 12). They are respectivelythe musculi pterygoideus and temporalis described in the adultXenopus by Grobbelaar (1924). The tentacular muscle ofthe larva has apparently no separate counterpart in the adult.With the complete disappearance of the tentacle at meta-morphosis, the muscle merges with the levator mandibulaeanterior of the adult.

Both fasciculi of the larval musculus levator hyoideus areconverted into the strong depressor mandibulae of the frog.In transforming specimens the arrangement is intermediatebetween that of the larva and adult. The quadrato-hyo-angularis, instead of being partly overlapped by the orbito-hyoideus, lies anterior to it and has now assumed an entirelydifferent position. It is more vertical than in the larva and,

HEAD OF XBNOPUS 169

with the disappearance of the processas muscularis, it moveson to the quadrate, from which it stretches to the end of themandible, being inserted on its ventro-lateral margin. Thefibres of the larger posterior fasciculus still ran to an insertionon the cerato-hyale as in the larva. In the young frog the twofasciculi are still as distinct as they are in the larva. Theanterior one, in addition to its connexion with the quadrate,also originates on the annulus tympanicus. The orbito-hyoideusof the larva has, however, now lost its connexion with thecerato-hyale, and stretches between the posterior extremitiesof the quadrate and mandible. It is continued backwards andupwards over the auditory region to the dorsal fascia, and lies(m.dep.man., figs. 19 a, b, PI. 13) externally to the museuli petro-hyoideus and cueullaris (m.pt.hy. and m.cu., figs. 19 a, b, PI. 13).

The petrohyoideus extends from the hinder part of the cristaparotica to the ventral surface of the crieoid cartilage. Thecueullaris arises close to the petrohyoideus, and in E a n a(Edgeworth, 1935) and adult Xenopus (Grobbelaar, 1924)it passes backwards to be inserted on the inner surface of thescapula. In the present preparations of young frogs, however,the musculus cueullaris passes over the inner surface of thescapula on to the clavicle near the union of the parts at theglenoid cavity.

Neither of these muscles is apparent in the larva of Xeno-pus and, due to the fact that they appear suddenly duringmetamorphosis, their evolution from the branchial muscles isdifficult of determination. Even in transforming specimens, inwhich the atrophying branchial arches are still evident as acompressed lateral process (bra., fig. 21 a, PL 13), both musclesare fully developed. At such a stage the remains of the lasttwo constrictores branchiales may still be seen passing betweenthe auditory capsule and the hyobranchial skeleton, just an-teriorly to the museuli petrohyoideus and cueullaris. Thecueullaris certainly seems to be separated from the dorsal endof the last constrictor branchialis, but the development of thepetrohyoideus is not so clear. During metamorphosis the larvalmuscle interpreted as a fused transversus ventralis IV andlevator arcuum branchialium IV atrophies to an irregular


bundle lying on the inner side of the diminishing branchialskeleton. The petrohyoideus lies more dorsally over this region,and posteriorly runs into this reduced muscle. The whole courseof the petrohyoideus at this time is reminiscent of the posteriorpetrohyoideus of other A n u r a , and in Xenopus it seems asthough it might be a modification of a muscle such as the lastlevator arcuum branchialium, or even in part of a transversusventralis IV, rather than simply of the posterior constrictorbranchialis which is still visible during metamorphosis. It istherefore assumed that as in both Urode la and the AnuraP h a n e r o g l o s s a , where the cucullaris and petrohyoideusare separated from the posterior branchial muscles (Edgeworth,1935), the cucullaris in Anura Aglossa has also a similarorigin. There is also some evidence that the petrohyoideus maybe derived from a muscle that in part resembles the levatorarcuum branchialium IV, in which case the single petrohyoideusof Xenopus would be comparable with the posterior petro-hyoideus muscle of certain other Amphibia.

The larval intermandibularis and interhyoideus muscles arenot subjected to any marked alterations at metamorphosis.The former is divisible into anterior and posterior parts, corre-sponding to the submentalis and submaxillaris respectively ofGrobbelaar's description of the adult musculature (1924). Theinterhyoideus is rather narrower than in the larva, and is re-ferred to as the subhyoideus by Grobbelaar.

There is no marked change in the laryngeal muscles atmetamorphosis. They are well developed even in young speci-mens measuring 10 mm. and in which the arytenoid cartilagesare just becoming demarcated in the connective tissue at theside of the glottis. The constrictor (con., fig. 20 a, PI. 13) anddilatator laryngis (dil., figs. 20 a, b, PI. 13) are both simple pairedmuscles inserted on the arytenoid cartilages. The insertionof the dilatator occurs above the constrictor laryngis, but asis demonstrated in figs. 20 a, b, PI. 13, its insertion is locatedslightly posterior to and not in front of the constrictor as isusual in Anura . While the constrictor is a short, more or lessobliquely vertical muscle, which takes its origin on the ventralside of the arytenoid cartilage, with the fibres of the two sides

HEAD OF XENOPUS 171

meeting mid-ventrally in some larvae, the dilatator laryngisis much longer and passes forwards from the postero-lateralangle of the chondrocranium.

Metamorphosis leaves the constrictor laryngis unchanged,but the dilatator becomes detached from the chondrocraniumand migrates on to the postero-medial process of the hyo-branchial skeleton {ppm., figs. 20, 21, PL 18). In the young frogit therefore originates from the posterior end of the postero-medial process near the ligamentum hyo-cricoideum.

A remarkable and interesting feature is the presence of a pairof muscles which are considered to be homologous with thelaryngei dorsalis of Urodele Amphibia. Eidewood (1897) wasof the opinion that in Xenopus there is an undifferentiatedconstrictor, to which he referred as the compressor glottides.In the present preparations of both larval and adult specimensof X. l aev is there is a distinct separation of muscles servingto constrict the larynx. The dorsal pair of muscles arises onthe arytenoid cartilages immediately above the insertion of themore ventral constrictor laryngis, but as is indicated in figs.20 a, i, PL 13, they are anterior to the insertion of the dilatator.They arch dorsally over the larynx, meeting in a mid-dorsalraphe.

While the arrangement of the laryngeal muscles seems topresent some considerable variation in Urode le Amphibia,the Anura appear to be characterized by the general occur-rence and constant arrangement of the constrictor and dilatatormuscles. Laryngei dorsales have been observed in variousUrode l a , but it is apparent that they are by no meansconstant, for Edgeworth (1935) mentions several genera inwhich they are lacking. The dorsal constrictors observed inX. l aev i s are strikingly similar to the laryngei dorsales of theUrode l a , and their presence may account for the moreposterior insertion of the dilatator muscle in this species. Thesemuscles have not been recorded so far in any other Anuran,and it is difficult to account for their presence in X e n o p u s .They may indicate a more primitive condition, or they maymerely represent a more obvious differentiation of the con-strictor muscle into dorsal and ventral parts.

172 NELLIE F. PATEESON

The throat muscles require but little mention for they havebeen fully investigated by Eidewood (1897). The geniohyoideusconsists of internal and external or lateral portions, insertedrespectively on the postero-medial and postero-lateral processesof the hyobranchial skeleton. There is no separation, as in theAnura P h a n e r o g l o s s a , of the lateral portion into internaland external muscles. Eidewood (1897) found that the genio-hyoideus of adult Xenopus arose near the mandibularsymphysis, the internus being anterior to the externus; but inthe present young specimens they seem to originate together atthe symphysis. The hyoglossal (hyog., figs. 20 a, b, PI. 13) wasnot observed to consist of the three portions described by Eide-wood (1897). In these young frogs the condition is essentiallyas in other Anura ; it arises from the anterior end of thepostero-medial process, near the crieoid; it meets its fellow ofthe opposite side, and the two run together through the hyo-glossal foramen to the floor of the mouth.

BLOOD-VESSELS.

Only two pairs of blood-vessels are relevant to the study ofthe chondrocranium and the cranial nerves. These are thebilaterally symmetrical internal carotid artery and the head vein(de Beer, 1926) or vena jugularis s. cardinalis anterior (Gaupp,vol. 2, 1899).

The internal carotid artery arises far back in the tadpole,and in coursing forwards it comes to lie laterally to theauditory capsule in a deep groove on the lower side of the cristaparotica. In this position it is immediately ventral to the headvein, the two vessels pursuing a more or less parallel coursethroughout. The head vein has to travel some considerabledistance beyond the level of the sinus venosus before joiningthe ductus Cuvieri, as the latter, owing to its unusual length,instead of being transversely directed, lies obliquely in apostero-anterior direction, and meets the sinus venosus at anangle.

In front of the auditory region (fig. 15, PL 12; fig. 27 6, PL 15)the two blood-vessels remain in juxtaposition for a shortdistance until the carotid artery moves mesially to supply the

HEAD OF XENOPUS 173

brain. The main stem of the internal carotid courses forwardbeneath the chondrocranium, where it is located mesially to thehead vein (fig. 11, PL 11; fig. 13, PL 12; fig. 276, PL 15). It sup-plies the palate, and when passing round the processas muscolarisof the palatoquadrate, it gives off a branch to the levatorhyoideus muscle. More anteriorly the artery lies close to theramus mandibularis V (figs. 7,8, PL 11) and divides to supply thetissues of the snout.

The arterial supply to the brain enters the cranium througha definite ventro-lateral carotid foramen, and immediatelydivides into two (fig. 12, PL 12; figs. 27 b, d, PL 15). The innerand rather more posterior branch is the arteria carotis cerebrals,while the outer and stouter branch is the arteria ophthalmica.

The arteria carotis cerebralis itself divides into anterior andposterior branches, as may be seen in fig. 27 d, PL 15. Theposterior artery (pea., figs. 13-16, PL 12), the R. posterior ofGaupp (1899) and Francis (1934), gives off a vessel to the lateralwall of the infundibulum, and a more posterior one anastomoseswith its fellow directly above the hypophysis. As in R a n a(Gaupp, 1899) and S a l a m a n d r a (Francis, 1934), shortly afterthis anastomosis the pair of posterior cerebral arteries unitebelow the medulla oblongata at the level of the cerebellum toform a median artery (ba., fig. 17, PL 12), the arteria basilaris(Gaupp, 1899), which extends along the ventral fissure of themedulla oblongata.

The anterior cerebral artery (aca., fig. 11, PL 11; fig. 12,PL 12) does not divide into dorsal and ventral parts as in theS a l a m a n d e r (Francis, 1934), but proceeds forwards overthe optic nerve, gradually approaching its fellow (aca., figs. 9,10, PL 11), with which it unites mid-ventrally at the anteriorend of the telencephalon. On its forward course it gives offblood-vessels to the brain; one branch in particular passeslatero-dorsally around the anterior end of the dieneephalon andapparently corresponds with the dorsal branch of the cerebralartery of the Sa lamander .

The arteria ophthalmica (opha., fig. 11, PL 11) is directedanteriorly and leaves the cranial cavity through the wideforamen for the nervus oculomotorius without approaching the


brain (fig. 11, PL 11; fig. 27 d, PL 15). It passes into the orbit{o'pha., fig. 10, PL 11), and, running antero-laterally, it enters theeyeball immediately above the nervus opticus, after havmgdivided as in the S a l a m a n d e r (Francis, 1934) to supply theeye-muscles and other tissues in the orbit.

The head vein is formed anteriorly by factors from the snoutand olfactory regions, where it seems to be homologous with theorbito-nasal vein of E a n a (Gaupp, 1899). Proceeding pos-teriorly it receives the venous blood from the levator hyoideusmuscle (l.h.vn., figs. 7, 8, PL 11), and is then located dorsallyto the musculi levatores mandibulae in the groove between theprocessus muscularis and the commissura quadrato-cranialisanterior (h.vn., figs. 7, 8, PL 11). Unlike the carotid artery, itdoes not pass round the processus muscularis, but pursues amore or less straight course to the orbital region where it issituated close to the subocular bar of the palatoquadrate. Inthe orbit it is joined by two veins. One, which corresponds withthe inferior orbital of R a n a , runs from the floor of the orbitand joins the head vein anteriorly. The other, the ophthalmicvein (o'ph.vn., figs. 9, 10, PL 11), is longer and passes back-wards from the dorsal part of the eyeball to join the main veinjust in front of the foramen for the nervus oculomotorius andnear the posterior limit of the subocular fenestra. Behind theorbit the vein passes to the ventral surface of the chondro-cranium, where it lies in a groove together with the ramusophthalmicus profundus V, the truncus hyomandibularis VII,and the ramus palatinus VII (fig. 13, PL 12; fig. 27 b, PL 15).

Posteriorly to the carotid foramen the venous blood from thebrain is collected into a large vein which passes between thetruncus supra-orbitalis and the rest of the ganglion pro-oticum(figs. 15,16, PL 12; fig. 27 d, PL 15). This cerebral vein is joinedamong others by a pituitary vein, which passes forwards fromthe hypophysis along the mesial side of the ganglion pro-oticumbut shows no signs of anastomozing with its fellow. As the com-ponent nerves of the ganglion pro-oticum diverge in passingthrough the wide foramen pro-oticum, the main cerebral veinalso makes its exit from the cranial cavity {cm., fig. 15, PL 12;fig. 27fc.PL 15) and joins the main head vein (h.vn., fig. 15, PL 12)

HEAD OF XBNOPUS 175

lying in the above-mentioned groove on the lower surface ofthe chondrocranium.

From the foregoing it is evident that, apart from a few minordetails, the main course of the internal carotid artery and thehead vein of X. l aev i s is in close agreement with the findingsof other workers on both Urode le and A n u r a n Amphibia.

THE CHONDROCBANIUM.

Within comparatively recent years information regarding thefiner details of the morphology of the head of representatives ofthe various vertebrate phyla has been accumulating rapidly,and in this connexion the impetus given to comparative cranialosteology, which has resulted in such interesting conclusionsregarding the evolution of the parts, has been largely due to theobservations of such workers as de Beer, Gaupp, Goodrich,Parker, de Villiers, and his students. The latter have beenresponsible more particularly for many exhaustive studies ofthe skulls of South African A n u r a .

It is to be noted, however, that in the Amphibia, whileobservations have been made on the development of the skullsof Urode la and Anura P h a n e r o g l o s s a , little informa-tion is to be found regarding the condition in the five genera ofAnura Aglossa . Edgeworth (1935), in his monograph onthe cephalic muscles of the vertebrates, describes the palato-quadrate of P ip a and of a larva of X. f raser i measuring17 mm. long. Some of his findings seem to be at variance withKotthaus's lengthy account (1933) of the chondrocranium ofX. l a e v i s . The present study, in addition to being an attemptat tracing the transition from the larval to the adult conditionin X. l aev i s , also endeavours to correlate the findings ofthese two previous investigators with the conclusions hereinarrived at concerning the construction of the chondrocranium.

Kotthaus (1933) has made a study of five different larvalstages, ranging in length from 7-4 mm. to 48 mm., and in agefrom 5 to 53 days. As the material for the present investigationwas collected locally in Johannesburg, where the larvae aboundin certain ponds and dams, no attempt was made to record their


ages, but the measurements of the preparations selected forstudy are as follows:

Stage.

1234567

Total Length.

5-7S mm.7

1016286049

Length to Cloacal Aperture.

2-0 mm.2-75 „4-7 „6-25 „

11-9 „29-8 „

20-25 mm. (metamorphosing)

Figure.

22 a22 623—272425a & 6

In addition to the above, serial sections were also made ofa larva which had just emerged from the egg, and of severalyoung frogs after decalcification of the skeletal elements.

Diagrammatic reconstructions such as those sketched in figs.22a,&, 23, PI. 13; figs.24, 25a,b, PI. 14; figs. 26a,b, 27a-d,Pl. 15were plotted graphically from drawings of the serial sections,and in some cases, as for instance in the chondrocrania shownin fig. 25 a, PI. 14; figs. 27 a-d, PL 15, wax models were also con-structed to check the accuracy of the graphical reconstructions.

It is apparent from the above short table of measurementsthat the first two stages at present under observation are bothyounger than Kotthaus's first stage, while Stage 3 seems to beintermediate between his first and second larvae. There arealso some slight variations in the sizes of subsequent stages, butthese differences are, however, hardly of any consequence toa study that endeavours to survey the whole development ofthe chondrocranium.

The chondrocranium of Xenopus is of the more primitiveplatybasic type described by Gaupp and Goodrich (1930). Thisis clearly demonstrated in the very early stages depicted infigs. 22 a, b, PI. 13, but is also evident even in larvae measuring10 mm. in length (fig. 23, PI. 13). Serial sections of a larva whichhad just emerged from the egg, and which like Stage 1 alsomeasured 5 mm. in length, revealed that the chondrocraniumwas not sufficiently differentiated from the connective tissueto allow of the determination of the parts. Therefore, no stageshowing the separate trabeculae cranii and parachordals was

HEAD OF XENOPDS 177

found, even although there was little difference in the externalappearance of the recently hatched larva and that used forfig. 22 a, PI. 13, in both of which the mouth was still closed.

In Stage 1, therefore, the trabecnlae (trab.) and parachordals(pcli., figs. 22 a, b, PL 13) have already fused, and the primitivefenestra hypophyseos (basicranial fenestra of Kotthaus, 1933)is very broad and filled by the hypophysis. Even at this earlystage, the fenestra is bordered anteriorly by the broad inter-trabecular plate (Up., fig. 22 a, PL 13), which itself is con-tinuous with the flat ethmoidal region in front. In larvaemeasuring 10 mm. long (Stage 3, fig. 23, PI. 13) the fenestra issomewhat smaller, and in these measuring 16 mm. it wasobserved that the membrane stretching across its floor wascompletely invaded by a single layer of cartilaginous cells. Thefenestra, therefore, closes rather earlier than is indicated inKotthaus's preparations, in which traces of it were still foundin larvae measuring 18-2 mm.

The carotid foramina are indicated for the first time byKotthaus in the 18-2 mm. long larva, so that it is not evidentif they have already been demarcated in his first stage larva.Among the present preparations the carotid foramina are estab-lished in larvae measuring only 10 mm. in length (i.e. Stage 3).These are only slightly older than Kotthaus's first stage, whichmeasured 7-4 mm., but as is evident in fig. 23, PI. 13, there isa considerable bridge of cartilage intervening between thefenestra hypophyseos (fb.) and the carotid foramen (/<?.).

The parachordals are rather short in X e n o p u s , and theyunite early to form the broad basal plate enclosing the noto-chord. The anterior end of the latter has already been reduceda little even in Stage 2 (fig. 22 b, PL 13), but its diminution isnot nearly so rapid as is supposed bj Kotthaus (1933), for as isshown in fig. 27 d, PL 15, the notochord still extends well forwardsand has the dorsal curvature described by Goodrich (1930) fortypical forms. Even in older forms with a total length of as muchas 60 mm., the notochord, although considerably reduced, maystill be traced up to the level of the posterior acustic foramen.

In early larval life (figs. 22 a, b, PL 13) the brain lies fullyexposed above the broad ethmoidal and intertrabecular plates.

NO. 322 N


There are no indications of lateral walls, which develop later(Stage 3, fig. 23, PL 13) as more or less vertical cartilages directlycontinuous with the trabeculae cranii. Thus, in these first tnreelarval stages the brain is not at any point roofed over by carti-lage. Anteriorly it lies in a shallow depression in the inter-trabecular plate, and the nervi olfactorii are continued forwardsover the dorsal surface of the flat ethmoidal plate to the olfactorysacs. As development proceeds, the side walls gradually archover the brain, but as they only meet anteriorly and posteriorly,a single large fronto-parietal fenestra remains dorsally. Thearrangement is, therefore, somewhat different to that obtainingin R a n a, where Gaupp (1896) describes the presence of taeniaetecti transversalis and median's, and the resultant division of thedorsal opening into a fenestra frontalis and a fenestra parietalis.In X e n o p u s , due to the absence of the taeniae tecti trans-versalis and medialis, the single fenestra represents a combina-tion of those featured in E ana . It would also appear from thestudies of de Villiers and his students that, with few exceptions(Bufo, Schoonees, 1930), the Ranid arrangement of this partof the chondrocranium is not evident in South African A n u r a.

The posterior fusion of the cartilages dorsally results in theformation of the tectum posterius (tp., fig. 24, PI. 14; fig. 27 a,PL 15), which was first observed in larvae measuring 28 mm.(Stage 5). As is consistent with the condition found in otherAnur a (Gaupp, 1896; Goodrich, 1930), there is also an anteriorfusion of the cartilages forming a dorsal arch (to., fig. 24, PL 14;figs. 27, a, d, PI. 15) over the anterior part of the brain. Thisanterior tectum is continued forwards, but never extends as faras the larval olfactory sacs. It occupies a position similar to thatof the epiphysial and paraphysial bars of Ami a and otherPisces (Goodrich, 1930), which, however, are transverse barsdeveloped from the supra-orbital cartilages.

The palatoquadrate of X. l aev i s is essentially similar tothat of X . f raser i (Edgeworth, 1935). In all larval stagesthere is a pronounced processus muscularis {mus.fr., figs. 7, 8,PL 11; figs. 22 a, b, PL 13; fig. 24, PL 14; figs. 27 a, c,Pl. 15)continuous with the commissura quadrato-cranialis anterior(c.q.c.a.) which is directed mesially to join the ethmoidal region.

HEAD OF XEKOPUS 179

The palatoquadrate passes backwards along the floor of theorbital region as a long subocular bar (s.o.b., fig. 9, PL 11; figs.22 a, b, 23, PL 13; figs. 27 a-d, PL 15) which ends in a prominentlateral process, the ventro-lateral process in Edgeworth's de-scription o f X . f r a s e r i . In Stages 1 and 2 the ventro-lateralprocess is faintly indicated and the subocular bar, although some-what slender posteriorly, nevertheless effects a fusion with it,the result being that the subocular fenestra is completely formedbetween the subocular bar and the trabecula cranii. Kotthaus(1933) has found that in his earliest stage (7-4 mm. long) thesubocular fenestra remains open posteriorly. It is therefore ofsome interest to note that, although the subocular bar is fullyestablished in Stages 1 and 2, in Stages 3 and 4 of the presentseries, with the greater elongation of the chondrocranium, theposterior part of the subocular bar becomes very slender. Thecartilaginous cells are only seen with difficulty at this point, andit almost appears as though the subocular bar and ventro-lateral process were united by connective tissue only. It isconsidered, however, that the subocular fenestra is in all stagescompletely enclosed.

The processus ascendens {ascpr., fig. 11, PL 11; fig. 13, PL 12;fig. 24, PL 14; figs. 27 a, c, PL 15) effects a fusion with the pilaantotica, so that there is a continuous cartilaginous wall betweenthe oculomotor and pro-otic foramina. The larval otic process,which is hardly evident in young larvae (fig. 23, PL 13), developsrather later and fuses with the auditory capsule (l.o.p., figs. 14,15, PL 12; fig. 24, PL 14; figs. 27, a, c, PL 15).

As there is no basal process (vide fig. 27 b, PL 15) the palatinenerve (pal.) and head vein Qi.vn.), which in the larva runparallel for some distance, lie freely in a ventral groove of thechondrocranium, and are not at any point underlain by a carti-laginous process of the palatoquadrate.

The quadrato-ethmoidal cartilage (q.e.c, fig. 6, PL 11; fig. 23,PL 13; fig. 24, PL 14; figs. 27 a, c, PL 15) is a slender bar extendingbetween the quadrate and the outer angle of the ethmoidalregion. It is not developed in the first two stages, but becomesevident in Stage 3 (10 mm. long), in which the rudiment of thetentacle makes its first appearance. About a third of the way


from the anterior limit of the quadrato-ethmoidal cartilagethe cartilaginous axial support of the tentacle projects laterally(e.tent, fig. 23, PI. 13; fig. 24, PI. 14; figs. 27 a, c, PI. 15), a con-dition which corroborates Edgeworth's view (1935) that theaxial cartilage is an outgrowth of the quadrato-ethmoidalcartilage. In his second stage larva, Kotthaus (1933) finds thatthe connexion between the quadrate and ethmoidal regions isincomplete, and he has concluded that the cartilage directedforwardly from the quadrate is the support for the tentacle.It follows, however, that if there is an incipient tentacle in thelarva represented in fig. 23, PL 13, there should be a slightlylonger tentacle in one measuring 12*3 mm., and the connexionbetween the quadrate and ethmoidal cartilage should be effected.

As the tentacle gradually elongates during larval life, its axialcartilage becomes concomitantly longer (e.tent., fig. 24, PI. 14;figs. 27 a,c, PI. 15). The present preparations have, however, notgiven full support to Edgeworth's statement (1935) that 'sub-sequently the posterior part of the quadrato-ethmoidal cartilagedisappears leaving the tentacle attached to the ethmoidalregion'. Parker (1876) has depicted this condition in the larva ofX. lae v i s , but these preparations of the same species indicatethat the quadrato-ethmoidal cartilage persists as a slender barwhich is still apparent just prior to metamorphosis in larvaemeasuring 60 mm. (Stage 6). During metamorphosis thequadrato-ethmoidal cartilage and also the tentacular cartilageare both absorbed, and neither is apparent at the end of thisprocess. It seems logical to suppose that, during metamorphosiswhen the jaws elongate, the connexion between the ethmoidcartilage and quadrate will disappear, so that for a time thetentacle will remain attached to the outer corner of the ethmoidalcartilage as Edgeworth maintains.

Authorities such as Gaupp (1896-1904), de Beer (1926,1937),and Goodrich (1930) have shown the importance of the positionof the various foramina in the elucidation of the parts of thechondrocranium. As a detailed study of the cranial nerves hasbeen made during the course of the present investigation (p. 198),some reference may here be briefly made to certain discrepanciesbetween the present findings and those of Kotthaus. In his

HEAD OF XENOPUS 181

account of the chondrocranium of X. laevis Kotthaus hasdealt more particularly with the positions of the nerves in thecranial cavity and their exits therefrom. The fact that he hasnot followed the entire course of each of the nerves may accountfor certain erroneous conclusions. He finds that in all stages,except the last, the nervi opticus and oculomotorius emergethrough a common foramen, but that in the last larval stagethey are separated. The preparations used for the present studyhave revealed that these two foramina are separated from eachother very early in larval life. Almost as soon as the lateralwalls of the chondrocranium are developed the pila metopticabecomes apparent. In one series of sections of a larva measuring10 mm. (fig. 23, PI. 13) the oculomotorius passed out throughthe narrower posterior part of the foramen opticum, but inanother series of a similar larval stage a narrow pila metopticaintervened between the two foramina. In all subsequent stagesexamined each of these nerves left the cranial cavity by its ownwide foramen. This arrangement is clearly shown in fig. 11,PI. 11, where the nervus oculomotorius is seen passing outposteriorly to the nervus opticus, and is also evident in figs.27 a, c, d, PI. 15, which are reconstructions of a younger larvathan that showninfig. 11, PI. 11. Kotthaus's account and figuresof the elements arising from the ganglion pro-oticum are some-what at variance with the present findings. It seems that he hasoverlooked the presence of the truncus supra-orbitalis, and hasthereby been misled in his interpretation of the other branchesof the trigeminal and facial nerves. He thus shows in his fig. 10a ramus maxillaris V, which, as will be explained later below,is considered to arise quite differently in X. l aev i s than thatof typical Anura . He also represents the ramus palatinus VIIas a lateral nerve passing dorsally over the ventro-lateral pro-cess of the palatoquadrate, whereas it maintains a ventralposition throughout. The nervus abducens, which is an extremelydelicate nerve passing from the cranium with the ramusophthalmicus profundus V, is represented as lying mesially tothe two branches of the trigeminus and as a nerve subequal inthickness to the rami of V and VII. It is therefore apparentthat there is little agreement between Kotthaus's views and the


present conclusions regarding the cranial nerves and their fora-mina. It is the opinion of the present writer that, immediatelyon passing out through the wide foramen pro-oticum, the truncussupra-orbitalis and ramus mandibularis V diverge from the restof the ganglion pro-oticum (vide figs. 14,15, PI. 12; figs. 27 a-d,PI. 15) and pass out dorso-laterally through what may correspondwith the superior trigeminal foramen of Kotthaus. The ramiophthalmicus profundus V, hyomandibularis VII, and palatinusVII pass out together through the wide ventro-lateral portionof the foramen pro-oticum (fig. 14, PI. 4; fig. 27 d, PL 15). Thedorsal and ventral parts of the foramen pro-oticum are directlycontinuous, and as in other Anura (Goodrich, 1930) there isno prefacial commissure.

The nervus acusticus penetrates the mesial wall of the audi-tory capsule through two foramina (acus., fig. 27 d, PI. 15), acondition which is evidently typical of the A n u r a , judgingby the observations of Goodrich (1930), Bchoonees (1930), duToit and de Villiers (1932), du Toit (1933), and de Vos (1935).Occasionally, as in Hemisus (de Villiers, 1931), only oneacustic foramen occurs, while H y l a m b a t e s (du Toit, 1930)is exceptional in having three. Confluent with the acusticforamina at an early stage is the endolymphatic canal (eZ/.+acus., fig. 23, PI. 13), the three becoming separate passages insomewhat older larvae (fig. 27 d, PI. 15).

The glossopharyngeal, vagus, and posterior lateral line nervesafter arising separately from the brain, unite within the cranialcavity (figs. 27 a, d, PI. 15). They therefore emerge togetherthrough the postero-ventrally situated foramen jugulare (J.jug.,fig. 27a,PI. 15), immediately lateral to which they form the largecomplex ganglion (g.v.), from which the several branches arelater separated.

Little importance attaches to the jaw region. The Meckel'scartilages are slightly curved rods (Meek., figs. 22 a, b, PI. 13;fig. 24, PI. 14; figs. 27 a, c, d, PI. 15), which project forwards fromthe quadrate below the superior labial cartilage formed by theanterior margin of the ethmoidal cartilage. The Meckel's carti-lages are joined by a single median inferior labial cartilage (ilc,figs. 22 b, 23, PI. 13; fig. 24, PI. 14), which is not present until

HEAD OF XENOPUS 183

the mouth opens (fig. 22 b, PL 13). It is noticeable in thediagrams that, compared with the whole chondroeranium, thejaws of the larva are very short. They are situated far forwards,articulating with that part of the quadrate in front of the orbit.

1. Me tamorphos i s .Apart from the ossification of the parts, the chief differences

in the conformation of the chondrocranium attendant on meta-morphosis are brought about by the rapid development of thenasal capsules and the backward movement of the jaws, so thatthe quadrate is located at the anterior level of the auditorycapsule.

As was previously mentioned, there is a wide dorsal fenestrain larval Xenopus , which at metamorphosis becomes roofed.over by the development of the fronto-parietal bones. Indescribing the skull of P h r y n o m e r u s , de Villiers (1930)discusses the question of this dorsal fenestra in relation to thefronto-parietalia, and finds that in the adult P h r y n o m e r u sthere is a large fontanelle between the two small lateral fronto-parietalia. That this is not an isolated occurrence is borne outby the observations of C. A. du Toit (1930, 1931) on Heleo-p h r y n e , de Villiers (1931) on Cacos te rnum and Brev i -ceps, and de Vos (1935) on Spe laeophryne , in each ofwhich there is a dorsal bridge of fibrous connective tissuejoining the pair of small fronto-parietalia.

The larger fronto-parietalia of P h r y n o b a t r a c h u s areobserved by du Toit (1933) to be separated mid-dorsally bya narrow strip of connective tissue, and he considers that, asthis condition obtains in so many different genera, as forinstance, in Arthroleptella, Anhydrophryne, Heleophryne, Bufo,and Hyperolius, it is typical of the Anura . I n H e m i s u s d eVilliers (1931) finds that there is a mid-dorsal fusion of thefronto-parietalia, and it is of interest to note in this connexionthat this somewhat atypical condition also obtains in Xeno-pus . The fronto-parietalia, which may be seen towards theend of larval life as paired dorso-lateral bones (fp., figs. 9, 11,PI. 11; figs. 12,14, PI. 12) coalesce mid-dorsally in the young frog,forming a continuous thin bone which in more mature specimens


(fig. 26 a, PI. 15) acquires a sagittal crest. It extends forwardsfrom the tectum posterius as a single bony sheet over the wholesurface of the brain, and at the nasal region is directly continuouswith what Gilchrist and von Bonde (1919) have termed the supra-ethmoid, a slightly arched membrane bone (seth., fig. 3, PL 9;fig. 4, PL 10; fig. 26 a, PL 15) which overlaps the paired nasals.A short distance behind the supra-ethmoid is a minute medianaperture (f.par., fig. 26 a, PL 15) through which, as transversesections reveal, the tractus pinealis passes. For this reasonWinterhalter (1931) has identified it with the foramen parietaleof Pisces, Stegocephalia, and Eeptilia.

The os en ceinture, a term applied by the majority of recentworkers to the bone encircling the sphenethmoid region,characteristically extends from the optic foramen to the nasalcapsule laterally and sometimes occurs ventrally between theparasphenoid and the solum nasi (de Villiers, 1931; du Toit,1933). In sections of young Xenopus the sphenethmoidregion is occupied by a very thin bone which forms the lateralwall of the cranium anterior to the optic foramen (o.e.c, figs. 26a, b, PL 15). It joins the fronto-parietalia with the cartilaginousfloor of the cranium in front, and rather more posteriorly it isconfluent with the parasphenoid. It is obviously homologouswith the os en ceinture, but it does not possess the large marrowcavities that characterize it in other Anura (de Villiers, duToit). The lateral wall of the cranium between the optic foramenand the pro-otic is evidently an ossification of the larval pilaantotica, and is therefore to be identified as the pleurosphenoid(pZs., fig. 26 a, PL 15), a term suggested by Goodrich for thisregion (1930).

In the majority of Anura the larval palatoquadrate de-velops into the adult processus pterygoideus, supporting thequadrato-maxillare and the paraquadratum. In the youngXenopus the processus pterygoideus (p.ptg., fig. 25, PL 14;figs. 26 a, b, PL 15) is a cartilaginous bar passing backwards fromthe antorbital process beneath the ventro-lateral surface of theeyeball, occupying a position similar to that of the subocular barof the larval palatoquadrate. It extends towards the quadrateand is roughly oval in transverse sections. In the subocular

HEAD OP XENOPUS 185

region only its mesial and dorsal surfaces are covered by bone,but behind the orbit it may be almost completely invested bythe pterygoid, which posteriorly has a ventral projection parallelto and almost meeting the processus coronoidens of the lowerjaw (cor., fig. 18 a, PI. 12).

In Xenopus the processus pterygoideus merges posteriorlywith the quadratum (fig. 18 b, PL 12), a relatively broad post-orbital process lying beneath the auditory capsule (quad., figs.18 a-c, PL 12). Duringmetamorphosisthequadratumisproduceddorsally into a prominent otic process (otp., fig. 25 b, PL 14) andventrally into an equally well-developed quadrate process(quad.). The otic process projects as far as the crista parotiea,but at this stage there is no fusion of these two parts, althoughin a lateral aspect (fig. 25 b, PL 14) the crista is seen to overhangthe otic process laterally. When metamorphosis is completed,the otic process articulates with the ventro-lateral wall of theauditory capsule, and anteriorly has a dorsal extension whichproceeds to the crista parotica, but its synostosis with the latteris not so marked as in the majority of the Anura P h a n e r o -glossa (de Villiers, 1930, 1931; du Toit, 1933; de Vos, 1935).

The remaining elements belonging to the palatoquadrate inthe majority of Anura are mainly of interest in Xenopuson account of their negativeness. The quadrato-maxillare(quadrato-jugal of Gilchrist and von Bonde, 1919), which ap-pears to be a feature of many Anura Phane rog los sa(Gaupp, 1896; de Villiers, 1930 et seq.) is absent in X e n o p u s .Gilchrist and von Bonde state that ' they (the quadrato-jugals)are very inconspicuous' and do not represent them in theirdiagrams. The maxillare (max.,figs. 26a, b, PL 15) iscontinuedforsome distance beneath the antorbital process, but it ends abruptlyand has no connexion with the quadrate, so that it is quiteevident that the quadrato-maxillare is lacking i n X e n o p u s .

de Beer (1926,1937) and Goodrich (1930) have explained theimportance attaching to the articulations between the palato-quadrate and the chondroeranium in the Tetrapoda, and in thisconnexion the presence of a basal process is of interest. Theformer author has shown that in the Anura Phane rog los sathere is no true basal process, but that there is a pseudo-basal


process posterior to the ramus palatinus VII. In the fore-going account of the chondrocranium of the larval Xenopusit has been shown that there is no basal process. During meta-morphosis the ascending process of the palatoquadrate dis-appears, a feature which is consistent with the development oftypical Anura (de Beer, 1926, 1937). The adult otic process(otp., fig. 26 a, PL 15) is a short broad bone which passes for-wards and inwards from the quadrate to meet the anterior marginof the pro-otieum (pro.). During and after metamorphosis theanterior end of the auditory capsule is connected to the basalplate by a ventral cartilage (ppc, figs. 18 a-c, PL 12; fig. 26 b,PL 15) which underlies the large ganglion pro-oticum (g.pro.). Itpasses downwards and outwards, coming into contact not onlywith the pterygoideus but also with the inner margin of thequadratum, near the junction of the latter and the processuspterygoideus (figs. 18, a-c, PL 12). The head vein Qi.vn., figs. 18a, b, PL 12) and the ramus hyomandibularis VII Qvym.) liedorsally to it, and as it lies posteriorly to the ramus palatinusVII, and consequently does not affect the course of this nerve,there can be no doubt that it is comparable with the post-palatine commissure of other Vertebrates, and therefore withthe pseudobasal process described by de Beer (1926, 1937).

As in other Anura (de Beer, 1926, 1937), the pterygoideusof Xenopus has a medial portion (m.ptg., fig. 18 c, PL 12;fig. 26 b, PL 15) which projects as a thin horizontal platebelow the eustachian tube, and anteriorly is in contact withthe post-palatine commissure.

The squamosum (paraquadratum of Gaupp and de Villiers)appears during metamorphosis as a slender membrane bone (prq.,figs. 18 a-c, PL 12; figs. 26 a, b, PL 15) lying lateral to the auditorycapsule and immediately above the plectrum (plec). It forks pos-teriorly over the middle ear, so that in sections (figs. 18a-c,P1.12)dorsal and ventral parts are evident and the ventral part becomesclosely apposed to the ventral part of the annulus tympanicus.

Gilchrist and von Bonde (1919), in their macroscopic examina-tions of the skull of X e n o p u s , have also remarked on theabsence of the palatinum. This, however, is a variable quantityeven in the Anura P h a n e r o g l o s s a , in some species of

HEAD OF XENOPTJS 187

which it has also been observed to be lacking (de Villiers, 1933;de Vos, 1935).

In regard to certain of these negative points, it is of interestto note that the quadrato-maxillare and palatine are alsoabsent in H e m i s u s , B r e v i c e p s , and P r o b r e v i c e p s(de Villiers, 1931, 1933). These, and other features that thesespecies may have in common with X e n o p u s, cannot, however,indicate any possible affinity, nor can they even be regarded asthe results of adaptations to similar habitats or habits, foras de Villiers (1931) has shown^ Hemisus is very markedlymodified for terrestrial life, while, as is well known, X en op usis a purely aquatic form.

Xenopus is somewhat exceptional among Annra inhaving a single median vomer (= praevomer, Broom, 1903,1935). The parasphenoid, which is regarded by Broom as thehomologue of the mammalian vomer, is one of the first bones tobe observed in X e n o p u s . It appears in larvae measuring28 mm. (Stage 5), in which its pointed anterior end occurs belowthe bulbi olfactorii, while its posterior extremity is located nearthe anterior limit of the notochord. It constitutes the somewhatthin-walled floor of the cranial cavity, and as it maintainspractically the same width throughout it does not possess thepostero-lateral processes that are so conspicuous in the Eanidtype of skull (jps., fig. 26 b, PL 15). During metamorphosis thepraevomer is developed along the ventral surface of the septumnasi, and from its inception is directly continuous with theparasphenoid, no line of demarcation being observed betweenthe two. In Teleostei, where a median praevomer (Goodrich,1930) is a typical condition, it is thought to represent a fusion ofa pair of embryonic structures, but i nXenopus no indicationof a paired origin was found even during metamorphosis. Infact, in sections it merely appears to be a forward prolongationof the parasphenoid along the floor of the olfactory capsule,and even though it is such a distinct bone (vom., fig. 26 b, PL 15)in the adult, there is a possibility that the median praevomerof Xenopus is merely an anterior part of the parasphenoid.

The lower jaw is typical of the Anura in the investitureof the Meckel's cartilage by only two membrane bones, the


angulare and the dentale. The former is the posterior crescenticbone (ang., fig. 18 c, PI. 12) which leaves the cartilage exposeddorsally and which is produced mesially into a well-marked pro-cessus coronoideus (cor., fig. 18 a, PI. 12) for the insertions of themusculi levatores mandibulae (lev. man., fig. 18 a, PI. 12). The den-tale, as in the majority of A n u r a, is a slender edentulous bone.

2. Ol fac tory E e g i o n .One of the most striking occurrences at metamorphosis is the

rapid, almost sudden transformation of the olfactory region.Throughout larval life the nasal sacs lie on each side immediatelyabove the lateral part of the ethmoidal cartilage, and justbehind the junction of the ethmoid and quadrato-ethmoidalcartilage. Not even in older larvae, such as Stage 6, were thecartilages of the adult nasal capsule foreshadowed.

In the larvae the internal nares tend to lie at a level anteriorto the widely open external nares, so that the former are locatedin front of the union of the palatal process of the palatoquadratewith the ethmoidal cartilage. In the very young larvae, beforethe mouth opens, neither of these apertures is established. Thethree cavities of the adult may be recognized, the laterallyplaced cavum medium (cav.med., fig. 6, PL 11) communicatingwidely with the cavum principale (cav.princ). The recessusmedialis of the cavum inferius is remarkable at this time forits size, and for the fact that, as in the majority of adult A n u r a,it is anteriorly situated and appears in sections even beforethe cavum principale. This may be due to the anterior positionof the internal nares in the larva, for in the adult X e n o p u sthe internal nares and also the recessus medialis are more pos-teriorly situated. The glandula nasalis medialis occurs betweenthe dorsal wall of the recessus medialis and the cavum principaleas they do in the young frog. The section represented in fig. 6,PI. 11, does not pass through the recessus medialis, but the glandsmay be seen lying immediately ventral to the cavum principale.

During metamorphosis the whole of the anterior part of theskull becomes complicated by the development of the cartilagesand additional chambers in the olfactory region. This region,'as well as the auditory capsule, has been thoroughly examined

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in various genera of South African Anura by de Villfers andhis students, and although he concludes (1930) that there is,on the whole, little variation in the size and shape of the nasalcavities, yet, as is to be expeeted, there are some slight differ-ences that seem to characterize the individuals.

In Xenopus the cartilages (fig. 25 a, PL 14) are broadlyarranged on a plan similar to that described by Gaupp (1896-1904) in E a n a , which has formed the basis of all more recentwork. The whole nasal region of Xenopus is projected up-wards and forwards in front of the lower jaw, and is remarkablefor the size of the cartilago alaris, which is outlined in fig. 25 a,PI. 14 (cart.al.). The cartilago alaris, the tectum and septum nasiall appear in sections anterior to the nasal cavities, and as thecartilago alaris extends as far back as the planum terminate, itconstitutes the main lateral cartilaginous wall of the nasalregion. Anteriorly it has a very narrow connexion with thesolum nasi just in front of the crista intermedia, and behindthis its inner concave surface is curved over the lateral surfaceof the cavum medium, the naso-lacrimal duet (n.l.d., fig. 2, PL 9)passing between it and the septomaxillare (sem.). There is onlyone prenasal cartilage, and as this is an antero-ventral extensionof the cartilago alaris, it must be regarded as being comparablewith the superior prenasal cartilage of E a n a . It is, however,ventral in position, and differs from that of E a n a and ap-parently from the majority of Anura in that it is flexed underthe nasal capsule and is continued posteriorly to fuse with thesolum nasi. This arrangement partly resembles that obtainingin Hemisus (de Villiers, 1931), in which there is likewise noinferior prenasal cartilage and in which the cartilago prenasalissuperior arises postero-ventrally from a large cartilago alaris.In H e m i s u s , however, it does not seem to establish a con-nexion with the solum nasi, so that in Xenopus it might evenbe regarded as a fusion of the two prenasal cartilages presentin E a n a . As in Hemisus , the prenasal cartilage (spc, figs.1-3, PL 9; fig. 25 a, PL 14) acts as the support for the premaxillare,which like the maxillare is toothed. It extends as far back asthe glandula intermaxillaris (g.i.m., fig. 3, PL 9) being situateddorso-laterally to it, but never imbedded in it. The lamina


superior and inferior are not quite normal when compared withthe Eanid formation. Following Gaupp's description (1904),the crista intermedia should separate the superior and inferiornasal cavities and the cavum medium should lie between itstwo laminae. In Xenopus the crista intermedia itself isnormal, but rather lateral in position, so that it is located (eris.,fig. 2, PI. 9) between the cavum medium and the recessusmedialis of the cavum inferius. The crista intermedia is notproduced into a conspicuous lamina superior above the cavummedium as in the majority of A n u r a . In H e m i s u s deVilliers (1931) has observed an unusually small lamina, superior,which, however, is normal in position. In X e n o p u s theusual lamina superior must be considered as being absent, aconclusion which is supported by the position of the septo-maxillare in this region (sem., fig. 3, PI. 9). It may be noted,however, that at this point a thin inwardly directed cartilage(he, fig. 3, PI. 9) is given off from the crista intermedia in frontof the planum terminale. It lies between the cavum principaleand the recessus medialis, and is, therefore, atypical of thelamina superior. Furthermore, the lamina superior is usuallydirected laterally from the crista intermedia, but in this case itis mesially directed. These peculiarities are perhaps to becorrelated with the fact that in Xenopus the recessus medialisof the cavum inferius is more posteriorly situated than in otherAnura previously investigated.

Posteriorly to this horizontal cartilage is another mesiallydirected cartilage. This is a thin horizontal rod occurringdorso-laterally over the isthmus near its junction with therecessus lateralis, and is evidently to be compared with theprocessus lingularis of E a n a (Gaupp, 1904).

The lamina inferior is a small posterior cartilage which fuseswith the planum terminale of the cartilago obliqua. Thelamina inferior (lam.inf., fig. 25 a, PI. 14) is dorso-laterally flexed,somewhat as in P h r y n o b a t r a c h u s (du Toit, 1933) andH y p e r o l i u s (du Toit and de Villiers, 1932). Together withthe planum terminale it forms in section a somewhat horse-shoe-shaped cartilage passing below the postero-lateral portionof the cavum medium and the naso-lacrimal duct. The septo-

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maxiilare forms a dorsal arch over this region and almost meetsthe lateral part of the lamina inferior. Bather more posteriorly,and after the disappearance of the cavum medium fromsections, the ductus nasolacrimalis is for some time almostencircled by the septomaxillare.

The nasal bones (nas., figs. 2, 3, PL 9) are large in Xeno-pus and almost meet mid-dorsally above the septum nasi.Anteriorly they are narrowed and lie above the tectum nasi.They then extend laterally over the cartilago obliqua, andbehind the planum terminale they rest on the planum antorbi-tale, bridging the gap between the anterior projection of thelatter and the tectum nasi.

3. Nasal Cav i t i e s .Some idea of the arrangement of the nasal cavities of the

young frog may be gained from an examination of figs. 1-3,PL 9; fig. 4, PL 10. The superior and median cavities are large,but the inferior in comparison is rather small. There is a smallvestibular region supported laterally hy the cartilago alaris. Thetwo 'Wulste' described by Gaupp (1904), and to which recentworkers seem to attach some importance, are both represented.Immediately behind the vestibulum there is a recessus sacci-formis communicating with both the infundibulum and thecavum medium, its relations with the latter being somewhatsimilar to those observed in Cacosternum by de ViUiers (1931).The recessus sacciformis (recsac, fig. 2, PL 9) is smaller thanthat of Ban a, and appears to be closely associated with thelateral wall of the ductus nasolacrimalis (n.l.d., fig. 2, PL 9).The two appear almost together in sections immediatelyposterior to and below the vestibulum, and both seem to effecta wide communication with the infundibulum. in Bana thetwo open close together into the cavum medium, but in X en o -pus they seem to be continuous with the infundibulum justanterior to the cavum medium, the aperture of the ductusnasolacrimalis being immediately behind that of the recessussacciformis.

The ductus nasolacrimalis (n.l.d.,ftg. 3, PL 9) passes backwardsover the outer surface of the planum terminale, with the postero-


lateral portion of the cavum medium (cav.med., fig. 3, PI. 9)situated laterally to it. It passes laterally over the planum ant-orbitale, and during metamorphosis communicates with Jieexterior in front of the lower eyelid by two apertures, theanterior of which is very inconspicuous. The presence of twoexternal apertures is, however, not an unusual occurrence, fortwo openings have also been recorded in H y p e r o l i u s bydu Toit and de Villiers (1932). In the young frog the anterioraperture disappears and the terminal part of the duct thenforms the lumen of the so-called 'tentacle' of the adult (n.l.d.,fig. 5, PL 10), the aperture being located at its free extremity.

As in E a n a, the plica terminalis is similarly associated witha horizontal fold behind the vestibulum, but the plica obliqua,which is well' developed and more posteriorly located than inE a n a , depends from the cartilago obliqua and the septo-maxillare. It has also been observed by de Villiers (1930,1931)to be similarly associated with the cartilago obliqua in P h r y n o-merus and Oacos te rnum.

The cavum medium (cav.med., fig. 2, PL 9) is large and ismore or less horizontal in anterior sections. It has a large rathervertical postero-lateral extension (cav.med., fig. 3, PL 9), thedorsal and ventral walls of which are very thin.

There is little to note regarding the cavum inferius. In com-parison with the other two chambers it is small, and its recessusmedialis (rec.med., fig. 3, PL 9), which in other Anura is welldeveloped and anteriorly situated, is smaller and more posteriorin position in X e n o p u s . A narrow recessus lateralis (rec.lat,fig. 3, PL 9) lies above the maxilla and continues as far backas the choane, where it forms a deep bay (smp., figs. 4, 5, PL 10)at the lower end of the cavum principale, thus correspondingwith the sulcus maxillopalatinus of E a n a (Gaupp, 1904).As may be observed from figs. 4 and 5, PL 10, Xenopus differsfrom E a n a and other Anura in having no marked eminentiaolfactoria.

A certain degree of interest attaches to the glands in thenasal region. The glands of Bowman and the glandula nasalismedialis are arranged similarly to those of E a n a (Gaupp,1904). In Xenopus the latter occurs between the cavum

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principale and the recessus medialis of the cavum inferius.Posteriorly to the recessus medialis it is located among thebranches of the nervus olfactorius, between the cavum princi-pale and the septum nasi. It seems to consist of two parts, amore anterior part opening into the floor of the cavum principale(d.g.m., fig. 1, PL 9), and a posterior portion which communicateswith the dorsal wall of the recessus medialis close to the level ofthe section shown in fig. 3, PI. 9. The glandula nasalis lateralisof E a n a is lacking in X e n o p u s , but the presence of an inter-maxillary gland is of interest, de Vos (1935) has indicated theabsence of this gland in H e m i s u s , Spe laeophryne , andin the P ip idae , its absence in the Anura Aglossa beingcorrelated with the aglossal condition. In Xenopus,however,there can be no doubt as to the identity of the well-developedgland (g.i.m., fig. 3, PI. 9; figs. 4, 5, PI. 10) which occurs be-tween the premaxillae. It opens on each side near the choane,about 80-90/x. behind the section represented in fig. 4, PL 10.Miiller (1932) has enumerated his reasons for considering thatthis is an intermaxillary rather than a pharyngeal gland, and hisconclusions are fully corroborated by the present observations.1

4. Aud i to ry Eeg ion .In his youngest larval stage, measuring 7*4 mm., Kotthaus

did not observe a cartilaginous ear-capsule, but he found asmall lateral protrusion of the parachordal which he describesas follows: ' Am Ende des ersten Drittels ihrer Lange zeigen dieParachordalia einen kleinen, mit der Spitze nach aussengerichteten Auswuchs; von diesem Fortsatz nimmt die Ver-knorpelung der Ohrkapsel ihren Ausgang und wiirde der " Com-

1 Since the above observations were made the writer has been able toprocure a copy of Foske's account (1934) of the nasal cavities of X .l a e v i s , of which none was obtainable in South Africa while the presentstudies were being prosecuted. The above description of the nasal cavitiesis, on the whole, in agreement with that of Foske, the main difference beingone of terminology. In describing the glands associated with the cavities,Foske has identified the one between the pre-maxillae as a palatal ratherthan an intermaxillary, which he considers is absent, and he has termedthe one opening into the cavum principale the glandula oralis interna, agland not previously recorded in Amphibia.

NO. 322 O


missura basi-capsularis anterior" (Gaupp, 1893) bei Eana ent-sprechen.' In the present series of preparations the paraehordalsof Stage 1 do not seem to be so long as those of Kotthau^'s firststage, and no trace of the commissura basi-capsularis was found,although the auditory sacs (as., fig. 22 a, PL 9) were conspicuous.In slightly older larvae (Stage 2) the floor of the auditory capsule(fac, fig. 22 b, PI. 13) is established and is in contact with themore elongate parachordal, presumably at a point comparablewith the commissura basi-capsularis anterior.

In Stage 3, which is only 2-6 mm. longer than the youngestspecimen examined by Kotthaus, the auditory capsules arefully developed and have effected their fusion with the para-chordal region (fig. 23, PI. 13). In this and all subsequent stagesthe inner ear was found to be completely enclosed in cartilageexcept, of course, at such points as the fenestra ovalis, theendolymphatic and perilymphatic canals, and the two foraminaacustica. The large anterior and posterior dorsal openingsdepicted by Kotthaus in his second stage were not observed.The wall of the capsule is admittedly thin between the ridgesfor the semicircular canals, but there is no trace of a gap ineither the anterior or posterior cupula even in larvae measuring10 mm. in length.

The middle ear, which has been fully investigated in the adultXenopus byde "Villiers (1932), makes its appearance duringmetamorphosis. An interesting stage in its development wasobserved during this process, when the eustachian tube wasseen to develop as a narrow passage between the quadrateand the pro-otic. The eustachian tube has, at this time, alumen of almost subequal diameter throughout; the vaultdescribed by Parker and confirmed by de Villiers (1932) is not,as yet, evident but may be seen in the young frog.

The plectral apparatus seems to arise at two separate centresand becomes continuous in young forms. As is shown in fig.25 b, PI. 14, the pars externa, which in the young frog consistsof obvious dorsal and ventral parts, has not yet been clearlydifferentiated. In sections (vide fig. 18 b, PI. 12) they are faintlyindicated in the tissues above and below the pars media plectri.Applied to the blind end of the middle ear (me., fig. 25 b, PI. 14)

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is a rounded cartilage which must be considered as the anteriorend of the pars media plectri (pnip., fig. 25 b, PL 14). At thisstage it is not connected with the rest of the plectrum, whichis composed of a fairly long cartilaginous rod divisible into thepars media and pars interna of the adult. The latter is ratherbroader than the median part and is applied to the fenestraovalis (J.o., fig. 25 b, PL 14).

de Villiers (1932) finds that in the adult the pars mediaplectri passes through the middle ear, dividing it into dorsaland ventral chambers, an occurrence which is unusual amongSouth African Anura . In the present preparations of meta-morphosing larvae and young frogs this condition does notobtain. In all of them the pars media plectri (jmip., figs. 18 b, c,PL 12) lies externally to the middle ear, which remains undivided.Xenopus is known to present many variable morphologicalfeatures, so that it may be suggested that the condition describedby de Villiers is either not constant or is only to be observedin older frogs.

The annulus tympanicus is poorly developed in metamor-phosing forms. Its ventral rim passes backwards some distance,but its dorsal rim is inconspicuous. Even in young frogs (fig.18 a, PI. 12) the dorsal rim (dat.) is less marked than the ventral(vat), and although it becomes rather better defined as develop-ment proceeds, the whole structure (see figs. 26 a, b, PL 15) neverbecomes quite annular as it is in E a n a ; the squamosum liesclose to the dorsal lip of the annulus, and in part serves as aboundary in this region. This condition probably explainsParker's description of the annulus tympanicus as a crescent-shaped structure.

In sections of a metamorphosing tadpole and of a young frogshortly after metamorphosis full confirmation has been ob-tained of de Villiers's conclusion (1932) that in Xenopus apars interna plectri and also an operculum are present as inmany other A n u r a . During metamorphosis the pars mediaplectri expands posteriorly into a broader pars interna, whichalmost fills the fenestra ovalis. A small cartilaginous operculumis faintly indicated 40/x. posterior to the extremity of the parsinterna, the intervening space being occupied by connective


tissue. In a very young frog which had just metamorphosed,the operculum (ope, fig. 19 b, PI. 13) was more obvious as acartilaginous piece between the upper and lower margins of thefenestra ovalis. At this stage the operculum is more obviouslyfused with the upper part of the auditory capsule, but its dorsaland ventral limits are still apparent. Between the pars internaplectri and the operculum the connective tissue, which is justbeginning to appear in the section represented in fig. 19 a, PI. 13(cnt.), has been somewhat reduced and now covers only 20/t.In rather older specimens the operculum becomes more closelyincorporated into the wall of the auditory capsule and the con-nective tissue joining it to the pars interna plectri is not soobvious. The condition of the sound-conducting apparatus inyoung specimens of Xenopus is therefore essentially similarto that of E a n a (Gaupp, 1904), and fully justifies de Villiers's(1982) conclusions regarding the identity of the structures inthe fenestra ovalis of the adult frog.

THE HYOBEANCHIAL SKELETON.

With the exception of the two early stages, the hyobranchialskeleton of the various larvae used in the present study differsbut little from that described and figured by Kotthaus (1933).Some variation in the rate of development is naturally to beexpected in these very early stages, so that while Kotthausfinds incipient developments of all four branchial arches inlarvae measuring 7-4 mm., the third and fourth being merelyslight lateral protrusions, in the first stage of the present seriesonly the first and second arches are established. In Stage 2(7 mm. long) the development of the arches has proceededfarther than Kotthaus's first stage, with the result that all fourarches have united and the branchial skeleton, although small,is actually in a condition similar to that of older larvae.

At metamorphosis certain of the parts of the hyobranchialskeleton undergo somewhat radical changes, but as these havebeen fully elucidated by Eidewood (1897, 1900) in his studiesof this and other species of the Anura Aglossa , it is onlynecessary to recapitulate certain of the more salient points.The features that have been emphasized by Eidewood as being

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characteristic of the Aglossa include the large hyoglossalforamen (kf., fig. 21 b, PL 13), which seems to appear after theabsorption of the basihyale and part of the basibranchiale. Itis bounded anteriorly by the anterior cornua, which in themetamorphosing stage (fig. 21 a, PI. 13) are joined by connectivetissue only (cnt), but which in the young frog (fig. 21 b, PL 13)effect a secondary cartilaginous fusion. The large postero-lateral processes are also characteristic of the Aglossa , andare considered by Eidewood (1897) to develop as secondary out-growths of the antero-lateral region of the branchial plate andnot from the remains of the branchial skeleton. This conclusionis based on the comparative study of the development in Pip aand X e n o p u s; but, on the other hand, a comparison of thetwo hyobranchial skeletons represented in figs. 21 a, b, PL 13,suggests the possibility of the origin of the postero-lateralprocess from the reduced branchial skeleton (bra., fig. 21 a,PL 13). The relative arrangement of the parts, and especiallythe position of the thyroid foramen (th.f., figs. 21 a, b, PL 13) areconstant in the two. One might, therefore, conclude that inX e n o p u s there is a direct correlation of the compressedbranchial skeleton and the postero-lateral process of the adult,although this does not seem to be substantiated by Eidewood'sobservations on P i p a.

During metamorphosis the thyroid foramen (ih.f.) graduallybecomes an open cleft, which in the young frog lies between theelongate postero-median (ppm.) and postero-lateral processes(ppl, figs. 18 a-c, PL 12; fig. 21 b, PL 13). The incipient postero-median processes are recognizable during metamorphosis (fig.21 a, PL 13), and their union with the larynx described by Eide-wood (1897) probably relates to later adult life, for in the youngfrogs herein examined they were only attached posteriorly tothe ericoid, the connexion seemingly corresponding with theligamentum hyo-cricoideum of E a n a (Gaupp, 1904).

The larynx agrees with Eidewood's description (1897). Itscartilages are noticeable even in early larval stages, and inyoung frogs the antero-ventral part of the annular ericoidcartilage (Ian., fig. 21 b, PL 13) becomes adherent to the pos-terior wall of the hyoid. The bronchial cartilages (br. pr.) are


comparatively longer than those observed by Eidewood in olderspecimens.

THE CRANIAL NERVES.

In demonstrating the anatomy of Xenopus to students thewriter has been at a loss to explain the innervation of the upperjaw. The maxillary and mandibular nerves are stronglydeveloped and subequal in thickness in typical AnuraP h a n e r o g l o s s a . In macroscopic examinations of Xeno-pus , however, while the mandibular nerve is easily traced, ithas not been possible to demonstrate the occurrence of a maxil-lary nerve similar to that of more typical genera. It was,therefore, in an endeavour to elucidate this point that thepresent microscopical observations of sections of both larvaeand young frogs were undertaken. As metamorphosis and itsresultant morphological changes has little effect on the distribu-tion of the cranial nerves, the following account is based chieflyon the condition in the larva. Certain changes are, of course,to be expected after metamorphosis, but as most of the remarksconcerning the larval arrangement are in general applicable tothe young frog, only the more striking variations between thetwo stages are indicated in the course of the discussion. As apoint of interest it may also be mentioned that a considerationof the larval arrangement of the nerves is significant in that itclearly indicates the similarities between the cranial nerves ofXenopus and those of certain Urode la (Norris, 1913;Escher, 1925; Benedetti, 1933; Francis, 1934) as well as thoseof the larva of E a n a (Strong, 1895).

In the reconstructions shown in figs. 28, 29, PI. 16, such nervesas the opticus, oculomotorius, trochlearis, and abducens havebeen omitted, as their inclusion would render the diagramsunnecessarily complicated. All these nerves except the abdu-cens, which on account of its proximity to the ramus ophthalmi-cus profundus V is difficult to indicate separately, are shownin figs. 27 a, c, d, PI. 15, and also in various diagrams of trans-verse sections.


Gangl ion P r o - o t i c u m .Approximately one-third of the way from the anterior end

of the medulla oblongata the facial and auditory nerves arisetogether as a large lateral mass. This close association of thesetwo nerves is characteristic of Pisces and Amphibia, and istherefore to be expected in X e n o p u s . The two are not easilyrecognizable as separate nerves even in sections of larvae, butthe fibres of the facialis issue more ventrally from the brainthan those of the acusticus, and as they pass antero-laterallythe former remains more mesial in position. Furthermore, asvan der Horst (1934) has shown in a larva slightly older thanStage 6 of the present series, they may be distinguished fromeach other due to the fact that the fibres of nervus facialis arenot so coarse as those of nervus acusticus.

Immediately in front of this mass, the anterior lateralis(ant.lat., figs. 27 a, c, d, PI. 15; figs. 28, 29, PL 16) emerges dorso-laterally by a single root, and runs forwards parallel anddorsal to the facialis nerve. In E a n a the anterior lateralis(dorsal VII) emerges with the facialis even in the tadpole, butin the U r o d e l a , as Strong (1895) has indicated, the twonerves are separate, so that in this respect Xenopus showsa closer approximation to the Urodele condition.

The nervus trigeminus arises just posteriorly to the separationof the facialis from the acusticus, and the three bundles offibres, anterior lateralis, trigeminus, and facialis, run anteriorlyparallel to each other, remaining quite distinct until about thelevel of the anterior part of the mesencephalon, where theybecome approximated to form the large ganglion pro-otieum(g.pro., figs. 27 a, b, d, PI. 15; figs. 28,29, PI. 16). At the anteriorborder of the cerebellum, and just before the ganglion pro-oticum is formed, the anterior lateralis divides into two (d.al.,v.al., fig. 17, PI. 12). The fibres in the dorsal part (d.al.) are notjoined by any other fibres from the ganglion pro-oticum, but re-maining distinct, they continue anteriorly as the truncus supra-orbitalis (ts., fig. 16, PI. 12), which, therefore, in Xenopus iscomposed of only lateral line fibres.

The fibres in the ventral portion of the anterior lateralis


(v.al., figs. 16, 17, PI. 12) have a closer association with theother components of the ganglion pro-oticum. They extendlaterally along the nervus trigeminus and intermingle with thefibres of the nervus facialis, just prior to or coinciding with themerging of these two nerves in the ganglion pro-oticum. Asimilar division of the anterior lateralis nerve (dorsal VII) intodorsal and ventral parts has been described in E a n a byStrong (1895) and in Siren by Norris (1913), in both of which,as in X e n o p u s , the dorsal half maintains a dorsal positionabove the trigeminus while the ventral half passes into thenervus facialis, supplying the lateral line fibres to the ramushy omandibularis.

In transverse sections the ganglion-pro-oticum (g.pro., figs.15, 16, 18 c, PI. 12) appears as a large oval mass, in which itbecomes increasingly difficult from behind forwards to distin-guish between the different fibres. In anterior sections of larvae,and especially after the separation of the truncus supra-orbitalis from the ganglion, the fibres are indistinguishable, afact also noticed by van der Horst (1934) in his preparations.It must be remarked, however, that in young frogs, where theganglion pro-oticum is large but relatively not so elongate asin the larva, the various component fibres are more easilytraced, probably due to the fact that the nerves separate soonerand almost simultaneously from the ganglion.

Nervus T r igeminus .

1. E a m u s M a n d i b u l a r i s .Norris (1913) has observed that in Siren the ramus mandi-

bularis V emerges through a foramen which is common to itand to the anterior lateral line nerve. In Xenopus a slightdifference was observed in this connexion between larval andadult conditions. Whereas in the larva the ramus mandi-bularis V and the truncus supra-orbitalis pass out throughseparate foramina, which, however, are situated close to eachother, in the young frog (fig. 18 a, PL 12) the nerves, after separat-ing from the ganglion pro-oticum, are directed anteriorly in thecranial cavity and again become approximated, so that they

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emerge together through a single foramen nearly opposite theorigin of the nervus opticus from the brain.

Strong (1895) has shown that in E a n a the rami ophthalmicusprofundus V and maxillo-mandibularis are ganglionated as theyleave the Gasserian ganglion, and it is considered that inXenopus an essentially similar condition obtains. The ramusmandibularis V is the first to separate from the ganglion pro-oticum after the dispersal of the anterior lateral line fibres,and it is slightly ganglionated as it proceeds dorso-laterally toits foramen. The ganglion of the ramus ophthalmicus profun-dus V remains in longer association with the nervus facialis,with which it emerges ventrally from the chondrocranium{of.-\-hym.-{-'pal., fig. 14, PI. 12). The fibres of the three nerves,ramus ophthalmicus profundus V, ramus hyomandibularis VII,and ramus palatinus VII, are, however, easily determined andseparate shortly after issuing from the chondrocranium (fig. 13,PI. 12).

In the larva the ramus mandibularis V (r.man., fig. 15, PL 12)separates from the ganglion pro-oticum almost in the sametransverse plane as the division of the truncus supra-orbitalisinto the ramus ophthalmicus superficialis and truncus infra-orbitalis. Shortly after emerging from the chondrocranium itgives off a branch (md^, fig. 12, PI. 12; fig. 28,P1.16) which passesout laterally in the direction of the truncus infra-orbitalis (io.).It applies itself closely to the latter, and passing beneath it,proceeds ventro-laterally over the dorsal end of the constrictorbranchialis II muscle and innervates some of the neuromasts.In the larva of E a n a , Strong (1895) observed three accessorytrigeminal branches which effected temporary fusions with thedorsal anterior lateral line nerve. These accessory nerves ofE a n a are stated to arise from a 'few large ganglion cells in thedorsal and mesial side of the trunk of the V, constituting theapex of the Gasserian ganglion' (p. 111). In Xenopus larvaethis ramulus of the mandibularis V seems to be directly com-parable with the middle accessory nerve of E a n a , for not onlydoes it come into contact with the lower ramus of the lateralline nerve as in E a n a , but it also arises dorsally from theramus mandibularis V which itself constitutes the upper part


of the ganglion pro-oticum after the truncus supra-orbitalis hasseparated from it. In young frogs a similar nerve (md.y, fig. 29,PL 16) is closely associated with the truncus infra-orbicalis, towhich it runs parallel and then ends in the tissues above themiddle ear.

The main stem of the ramus mandibularis V (r.man., fig. 13,PL 12) continues forwards and outwards in a deep lateral grooveof the chondrocranium, and opposite the optic foramen it givesoff another small branch (md.2, fig. 28, PL 16) which on its courseanteriorly passes laterally and ventrally to the floor of the orbit.It is located immediately below the truncus infra-orbitalis, withwhich, however, it does not seem to anastomose. Owing to itsposition and distribution it may nevertheless be comparablewith the innermost accessory trigeminal branch of Ban a.

In front of the orbit the ramus mandibularis V broadens outover the dorsal surface of the musculus levator mandibulaeanterior. More anteriorly, in the deep lateral groove betweenthe commissura quadrato-cranialis anterior and the processusmuscularis of the palatoquadrate, the ramus mandibularis V(r.man., fig. 7, PL 11) lies between the musculus levator mandi-bulae posterior and the levator muscle of the tentacle. Whilein this position the nerve supply to the levatores mandibulaeis given off from the ramus mandibularis (cf. fig. 7, PL 11).

Anteriorly, when it is situated between the quadrato-ethmoidal(q.e.c.) and Meckel's cartilages (Meek.), the ramus mandibularisV (r.man., fig. 6, PL 11) divides into several branches. Onebranch (md.3, fig. 28, PL 16) passes ventro-laterally to the skin,and is perhaps to be compared with the ramulus labialis ofSiren (Norris, 1913). Two other small branches later re-uniteto form the nerve (md.^ fig. 28, PL 16) to the tentacle. Of theremaining two branches, md.6 passes latero-ventrally round themargin of the Meckel's cartilage and supplies the musculusintermandibularis, while md.5 continues forwards lying dorso-laterally to the Meckel's cartilage. It divides anteriorly intoseveral branches which pass over the inferior labial cartilagesand innervate the tissues of the lower lip. These two nerves areidentical with the rami mandibularis inferior and superior ofP r o t e u s as described by Benedetti (1933), and also bear some

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resemblance to the ramuli intermandibularis and mandibniarfsexternus in Norris's account (1913) of the nerves of Si ren .

In the young frog the ramus mandibularis V is a very thicknerve, which after leaving the cranium is directed laterally andventrally, and in passing over the museuli levatores mandibulaeon its ventro-lateral course it supplies both these muscles. Itsanterior branches (md.g_s, fig. 29, PL 16) are similar to those ofthe larva, with the exception that md.5 and md.6 are given offafter the main ramus has passed to the ventral surface of themandible, with the result that md.5 is more ventrally situatedthan it is in the larva.

From the foregoing it is evident that the ramus mandi-bularis V of X e n o p us is composed largely of general cutaneousfibres together with some motor fibres, and that it correspondswith the mandibular portion of the maxillo-mandibular stemin other Amphibia. As is shown in figs. 28, 29, PL 16, there isno separation of a ramus maxillaris from this stem as is usualin A n u r a, the supply to the upper jaw being drawn mainlyfrom the ramus ophthalmicus profundus V.

2. E a m u s O p h t h a l m i c u s P ro fundus V.After the ramus mandibularis V has been given off from the

ganglion pro-oticum, the mixed fibres that remain form a largebundle which passes out ventrally from the chondrocranium(op.+hym.-\-j>al., fig. 14, PL 12). In this mass the large gan-glionated cells of the ramus palatinus VII are distinguishedventrally, while the ramus hyomandibularis VII is lateral andthe ramus ophthalmicus profundus V is located dorso-mesially.The three ganglia therefore emerge together, and in passinganteriorly are situated in a marked ventral groove (fig. 13, PL 12)in which they almost immediately separate from each other.

In the young frog the arrangement is otherwise. All thenerves are dispersed almost simultaneously from the ganglionpro-oticum; the ramus palatinus VII (pa?., figs. 18 a, b, PL 12)passes directly through its own ventral foramen; the ramushyomandibularis VII (jkym.) is directed antero-laterally betweenthe floor of the auditory capsule and the quadrate. There thenremain at different levels in the cranial cavity the ramus


ophthalmicus V (op.), the ramus mandibularis V (r.man.), andthe truncus supra-orbitalis (ts.), which all emerge rather moreanteriorly. The two latter, as has been explained previously, passout through a common foramen; the ramus ophthalmicus pro-fundus V has a separate foramen which is located immediatelyantero-ventrally to that for the ramus hyomandibularis VII.Immediately on leaving the cranial cavity, and before enteringthe orbit, the ramus ophthalmicus profundus V of the youngfrog becomes associated with the nervus oculomotorius in sucha way that it passes between the two branches of III. In thelarva this association occurs more anteriorly in the orbit as italso does in the Sa l amande r (Francis, 1934). The additionalanastomoses between the ramus ophthalmicus profundus V andthe nervi oculomotorius and trochlearis, shortly to be described,are more easily followed in sections of larvae than in prepara-tions of young frogs.

The ramus ophthalmicus profundus V of the larva is at firstsituated ventrally as in E a n a (Strong, 1895). It is a stoutnerve composed of general cutaneous fibres, and on its courseanteriorly it gradually assumes a more dorsal position. At firston entering the orbit (op., figs. 9, 10, PI. 11) it may be observedabove the subocular fenestra and in more anterior sections itoccurs near the commissura quadrato-cranialis anterior. Theposition of this nerve is essentially similar to that described inSiren by Norris (1913), where it at first runs in the gap betweenthe orbito-sphenoid cartilage and the base of the petrosal, andmore anteriorly it lies between the orbito-sphenoid cartilageand the temporal muscle. It also resembles the profundus nerveof the S a l a m a n d e r , which is described by Francis (1934)as lying mesially to the levatores mandibulae muscles as itpasses forwards to the orbit.

In the larval Xenopus , while it is situated over the sub-ocular fenestra and still some distance behind the eyeball, theramus ophthalmicus profundus V becomes closely connectedwith the nervi oculomotorius and abducens. The latter, whichas in most Amphibia arises ventrally in the same transverseplane as the IX-X complex, is in close contact with the ramusophthalmicus profundus V when it leaves the ganglion pro-

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oticum. They run forward together, only separating in theorbit, where the abducens is observed as a very fine nerve (VI,figs. 9, 10, PL 11) which proceeds for some distance ventrallyto the rectus externus eye-muscle before entering it. Francis(1934) and Edgeworth (1935) both mention that in Amphibiathe abducens also supplies the museulus retractor bulbi. InX e n o p u s , owing to its fineness, the abducens is difficult totrace beyond the rectus externus muscle, but in older larvaeand young frogs a few of its fibres seem to extend as far as theretractor bulbi, so that it is distributed similarly to that ofmore typical forms.

After emerging through its own separate foramen the nervusoculotomorius of the larva travels outwards and forwards untilit lies immediately dorsal to the ramus ophthalmicus profundusV. Asinmany other Amphibia it then divides (III, fig. 10, PL 11;fig. 27 a, PL 15), and its two branches apply themselves closelyto the ramus ophthalmicus profundus V lying in its bifurcation.The superior branch of the oculomotorius passes to the superiorrectus eye-muscle (r.s., fig. 9, PL 11) entering it ventrally. Im-mediately this branch of the oculomotorius has passed over thedorsal surface of the ramus ophthalmicus profundus V, thelatter gives off a short dorsal twig which anastomoses witha downwardly directed branch of the nervus trochlearis.

The inferior branch of the oculomotorius is the stouter andassociates with a ventral branch of the ramus ophthalmieusprofundus (pp.v fig. 28, PL 16) with which its fibres interminglebefore the two nerves again separate. The inferior ramus of IIIthen supplies the recti internus and inferior and the obliquusinferior eye-muscles. The delicate branch of the ramus ophthal-micus profundus V mentioned above (op.x) divides into two evenfiner branches, one of which anastomoses with a twig from thenervus trochlearis, while the other, following the course of thenervus opticus, enters the eyeball. Norris (1913) has alsodescribed a similar association of the oculomotorius and a branchof the ramus ophthalmicus profundus V, the ramulus ophthal-micus profundus minor, and suggests that it probably constitutesa superior ciliary nerve. In S i ren , however, this nerve isa dorsal branch of the ramus ophthalmicus profundus V, whereas

206 NELLIE F. PATEKSON

in Xenopus the branch which anastomoses with the oculo-motorius passes off ventrally from the main stem. This portionof the larval nervous system is perhaps more consistent withthe arrangement observed in E a n a (Gaupp, 1899) or in theS a l a m a n d e r , in the latter of which Francis (1934) hasdescribed a ramus communicans from the oculomotorius to theramus ophthalmicus profundus V, and close to its point oforigin a ciliary ganglion is formed. Francis further states thata twig from the ramus inferior III passes among the fibres of themusculus retractor bulbi and may continue to the eye, enteringthe ' sheath surrounding the optic nerve'. In the S a l a m a n d e rthis nerve constitutes an inferior ciliary, a superior ciliary nervealso being present. In Xenopus the superior ciliary nervewas not observed, and although a ciliary ganglion is not evidentalong the course of the nerve that enters the eyeball, it is con-cluded that it is comparable with the inferior ciliary of theS a l a m a n d e r and of typical Anura .

Shortly after entering the orbit, the ramus ophthalmicusprofundus V gives off a thin dorsal branch (op.2, fig. 28, PI. 16).This divides into at least three very fine twigs, the ventral oneuniting with the nervus trochlearis, while the other two supplythe subcutaneous tissue behind and above the eye. From itsorigin and distribution it would seem that this nerve correspondswith part of the ramulus ophthalmicus profundus minor ofSi ren , where according to Norris it supplies the skin aroundthe eye and also comes into contact with the nervus troch-learis.

The main ramus ophthalmicus profundus V pursues anantero-dorsal course, and in front of the anterior limit of thebrain it divides into at least four thin but quite obvious branches.The most dorsal of these (op.A, fig. 28, PL 16) proceeds latero-dorsally over the chondrocranium and enters the nasal capsuleas shown in fig. 8, PI. 11. This constitutes the ramulus nasalisinternus, which in the larva passes dorsally over the olfactorynerve towards the middle line. Anteriorly it passes to the innerside of the olfactory sac and gives off a small ventral branchwhich pierces the ethmoidal cartilage and ends in the connectivetissue below. The main ramulus nasalis internus is continued


anteriorly in front of the nasal region and terminates in thetissues of the snout.

The ramulus nasalis internus arises similarly in the youngfrog. It may be seen entering the nasal capsule in fig. 5, PL 10(int.nas.), and shortly afterwards it divides into two. Thethinner lateral branch passes forwards and outwards over thedorsal surface of the cavum principale and leaves the nasalcapsule (int.nas.!, fig. 3, PL 9), passing between the cartilagoobliqua and the nasale. Anteriorly to this it lies dorsally to theseptomaxillare and ends in the plica obliqua. The medial branch(inf.n«s.2,fig.3,P1.9; fig. 4, PL 10; fig. 29,P1.16) is a stout nerve,which after passing dorsally and mesially around the cavumprincipale lies between the septum nasi and glandula nasalismedialis. As in E a n a (Gaupp, 1899) it makes its exit fromthe nasal capsule by the fenestra naso-basilis and comes intocontact with the glandula intermaxillaris. It passes over thesuperior pre-nasal cartilage and premaxilla (int.nas.2, figs. 1, 2,PL 9) before dividing into several branches which terminate inthe glandular skin at the extremity of the snout. In some youngfrogs in addition to the above, the glandula nasalis medialis wasobserved to receive a supply from a fairly conspicuous ventrallydirected branch of the ramulus nasalis internus before it dividedinto its lateral and medial branches.

The other three terminal branches of the ramus ophthalmieusprofundus V of the larva (op.3, op.5, and op.s, fig. 28, PL 16)pursue a more or less parallel course anteriorly, there beingtemporary associations between them as is indicated in fig. 28,PL 16. Judging by their distribution they are to be correlatedwith the terminal ophthalmic branches of Siren (Norris, 1913)and P r o t e u s (Benedetti, 1933) rather than with those of theAnura . In comparing them with the nerves of S i ren , op.3and op.4 of the latter are respectively op.a and op.5 of Xeno-pus , in which case op.6 is identified as a ramulus nasalisexternus. A ramulus nasalis externus has been observed byBenedetti (1933) to be similarly separated from the ramusophthalmieus profundus V in P r o t e u s , and in all threegenera it innervates the skin lateral to and in front of the nasalregion. It is to be noted, however, that in the majority of

208 NELLIE F. PATBRSON

Amphibia the ramulus nasalis externus gives off a branch whichenters the nasal capsule, but this was not observed inXenopus .

After the anastomosis between the ramulus nasalis xternusand op.5 has been effected, a thin nerve (op.7, fig. 28, PI. 16)separates ventrally, and passing directly downwards over theanterior margin of the commissura quadrato-cranialis anterior,it becomes associated with an anterior branch of the ramuspalatinus VII below the ethmoidal cartilage. This nerve isprobably merely an anterior contmuation of op.5, and the wholenerve is to be regarded as homologous with op.4 of S i ren ,that is, with the ramulus palatinus profundus. This anasto-mosis between the ramus ophthalmicus profundus V and theramus palatinus VII is even more obvious in X e n o p u s duringmetamorphosis, when the ramulus (op.7, fig. 29, PL 16) forms astout nerve passing obliquely through the antorbital cartilageto join the ramus palatinus VII. In sections of young frogsthe ramulus communicans ad VII was more difficult to trace.A thin nerve was given off as in metamorphosing specimens,but its connexion with the ramus palatinus VII was not deter-mined with certainty.

The remaining branch (op.3, fig. 28, PL 16) is considered torepresent the ramus maxillaris. In Anura this branch isusually a well-defined nerve originating from the maxillo-mandibular trunk, but in Urodela its origin is variable. InSiren Norris (1913) has ascertained that the truncus infra-orbitalis is composed of general cutaneous fibres in addition tothe lateral line fibres. The general cutaneous fibres whichcomprise the ramus maxillaris are therefore separated from thetruncus infra-orbitalis in Si ren; the lateral line fibres innervatethe infra-orbital neuromasts as does the truncus infra-orbitalisin X e n o p u s . The arrangement in Xenopus seems toapproach more closely that of P r o t e u s , in which Benedetti(1933) has found that the truncus infra-orbitalis consists onlyof lateral line fibres, the general cutaneous fibres which formthe ramus maxillaris being derived from the ramus ophthal-micus profundus V, in a manner similar to the course of op.3in X e n o p u s . The arrangement in the larval S a l a m a n d e ris in essentials similar to that of S i ren , the ramus buccalis

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being connected with the lateral line nerve, but in the adult(Francis, 1934) the ramus maxillaris resembles that of A n u r a .

It is therefore evident that in the larval stage the arrange-ment of the branches of the ramus ophthalmicus profundus Vof Xenopus is intermediate between that of Siren andP r o t e u s . All the main ophthalmic profundus branchesdescribed by Morris are present in X e n o p u s , but in additionthe ramus maxillaris occurs as a branch of the profundus asit is in P r o t e u s .

A comparison between the larval arrangement and thatoccurring during metamorphosis is also of some interest. Fromfig. 29, PL 16, it is apparent that the ramuli ophthalmicus pro-fundus minor (op.^), the nasalis intemus (op.4), and the ramuluscommum'cans ad VII (op.7) arise from the main ophthalmictrunk much as they do in the larva. The ramulus nasalis ex-temus (op.e) and the ramus maxillaris (op.3) are situated closertogether, the latter separating in front of the branch ana-stomosing with VII, and not posterior to it as in the larva.The Harderian glands which develop during metamorphosis areinnervated by a special nerve (op.8, fig. 5, PL 10; fig. 29, PL 16)which passes laterally from the main ramus immediatelyanterior to the ramulus nasalis intemus.

Nervus F a c i a l i s .As in many Anur a, the geniculate ganglion is not separated

from the Gasserian ganglion, but it becomes closely associatedwith the trigeminus and forms the ventral part of the con-spicuous ganglion pro-oticum. As has been previously observed,the nervus facialis emerges from the medulla oblongata with theacusticus, from which it separates as a small but distinct bundlepassing forwards between the brain and the inner wall of theauditory capsule (VII, fig. 17, PL 12). In sections of both larvaeand young frogs the nervus facialis (VII, figs. 28, 29, PL 16)may be traced for at least 6G0-700JU. as a separate nerve lyingimmediately ventrally to the nervus trigeminus. It eventuallyapplies itself to the ventral surface of the latter after receivingsome lateral line fibres from the anterior lateralis nerve. Justas the latter is identical with the dorsal VII of the larva of

NO. 322 P


E a n a , the ventral nerve which emerges with the nervusacusticus corresponds with the ventral VII in Strong's descrip-tion of E a n a (1895).

Eegarding the derivation of the general cutaneous fibres ofthe facialis nerve in Amphibia, it is a generally accepted fact thatthese enter the facialis by way of the ramus communicans. Thisis probably also their usual origin in Xenopus , for a ramuscommunicans occurs in both larvae and adults. It has alsobeen suggested by van der Horst (1934) that 'in the ganglioniccomplex fibres from the trigeminus might join the hyoman-dibular nerve, and the same might occur where the greater partof the sensory facialis fibres pierces through the descendingtrigeminus, though it would be difficult to prove this'. Norris(1913) finds that in Siren the ramus jugularis VII may receivegeneral cutaneous fibres from the ramulus malaris of the ramusmandibularis V, and also that there is a small bundle of generalcutaneous fibres entering the brain near the motor root of VII.It has already been proved by van der Horst (1934) that thelatter does not occur in X e n o p u s , and during the course ofthe present investigation no anastomoses were observed betweenthe ramus mandibularis V and the ramus hyomandibularis VII.It therefore seems that in Xenopus , apart from van derHorst's suggestion, the only apparent source of the generalcutaneous component of the nervus facialis is through the ramuscommunicans.

1. Truncus S u p r a - o r b i t a l i s .Xenopus , in common with other Anura , has only a

single root to the anterior lateralis nerve, and this is regardedby van der Horst (1934) as being comparable with the ventralroot of the lateral line nerve of Pisces and Urode la . Theanterior lateralis nerve is quite distinct, and in the foregoingremarks attention has already been drawn to its division intodorsal and ventral parts in the vicinity of the ganglion pro-oticum. While the fibres of the ventral half lose their identityin merging with the nervus facialis, the dorsal division remainsdistinct and is easily traced anteriorly. In the ganglion pro-oticum of both larva and adult it maintains a dorsal position

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above the trigeminus, and there is no indication of its beingjoined by any cutaneous fibres from the latter.

The supra-orbital trunk is the first to separate from theganglion pro-oticum. It emerges through a wide dorso-lateralforamen just anterior to the mesencephalon (ts., fig. 16, PL 12).As it passes outwards it divides sharply into dorsal and ventralnerves. The dorsal branch (so., figs. 9-11, PL 11; figs. 12-15,PL 12) represents the ramus ophthalmicus superficialis of thelarva of E a n a (Strong, 1895) and of Siren (Morris, 1913).It passes anteriorly maintaining a dorsal position throughout.It gives off short posterior and anterior nerves to the skinbehind the nasal region, and anteriorly it occurs above theterminal branches of the ramus ophthalmicus profundus V,innervating the supra-orbital series of sensory organs.

The other branch of the supra-orbital trunk (io., figs. 12-15,PL 12) passes in a ventral direction until it reaches the lateralneuromasts. It then pursues an antero-lateral course im-mediately below the eye, giving off a series of twigs to the infra-orbital sensory organs and terminating in the neuromasts ofthe upper jaw. There is no doubt as to its identity with thelateral line part of the truncus infra-orbitalis of Siren (Norris,1913). It is also homologous with the ramus bucealis of thelarva of E a n a , which has been shown by Strong (1895) tobe composed of lateral line fibres innervating the infra-orbitalseries of sensory organs.

2. E a m u s H y o m a n d i b u l a r i s V I I .After the ramus mandibularis V has separated from the

ganglion pro-oticum, there remain mixed fibres received fromthe anterior lateralis, the trigeminus, and facialis nerves. Thelateral line fibres received from the ventral division of theanterior lateralis are difficult to trace in the ganglion pro-oticumof the larva, but in the young frog their passage is more easilydetermined. After spreading laterally over the trigeminus{v.al., fig. 16, PL 12) they are directed medially and eventuallypass into the ramus hyomandibularis VII, which, therefore, asin E a n a (Strong, 1895), in addition to its motor and generalcutaneous fibres, also receives fibres from the lateral line

212 NELLIE P. PATERSON

system. The rami ophthalmicus profundus V and palatinus VIIreceive no fibres from the anterior lateralis nerve, the formerbeing composed of general cutaneous fibres and the lattei ofcommunis fibres.

In the larva the ramus hyomandibularis VII is a broad nerve(hym., fig. 13, PI. 12) which separates laterally from the ganglionpro-oticum. As it assumes a more lateral position (hym., fig. 11,PL 11; fig. 12, PI. 12; fig. 27 b, PL 15) the head vein (h.vn.) runsbetween it and the more medially placed ramus palatinus VII(pal.). The ramus hyomandibularis VII proceeds in an antero-lateral direction, lying at first immediately below the subocularbar of the palatoquadrate where it is joined by the ramus com-municans from the IX-X complex. In front of this it takes upa position ventro-lateral to the subocular bar (hym., fig. 9, PL 11),and when the latter joins the processus muscularis, the ramushyomandibularis VII is located above the lateral part of thecerato-hyale. While in this position, it lies mesially to theorbito-hyoideus muscle, to which it sends at least two smallbranches. In passing round the articular surface of the cerato-hyale, the ramus hyomandibularis VII divides into two. Themore ventral branch (r.jug., fig. 28, PL 16) corresponds with theramus jugularis of Siren (Norris, 1913) and of S a l a m a n d r a(Francis, 1934). It passes ventrally close to the inner surfaceof the orbito-hyoideus muscle to which it supplies a few fibres.It also gives off a nerve which passes forwards to the quadrato-hyoangularis muscle, the main stem (r.jug., figs. 7, 8, PL 11)then proceeding ventrally between the orbito-hyoideus muscleand the lateral surface of the cerato-hyale, eventually applyingitself to the ventral surface of the musculus interhyoideus.

The dorsal nerve (r.ment, figs. 7, 8, PL 11) lies just ventro-laterally to the processus muscularis of the palatoquadrate(mus.pr.), between the orbito-hyoideus and quadrato-hyo-angularis muscles. On a level with the origin of the formermuscle on the processus muscularis, it divides into two. Onebranch (m.ext., fig. 6, PL 11) passes dorsally along the quadrato-hyoangularis muscle (dep.man.) and divides anteriorly to supplythe oral sensory organs near the base of the tentacle. The otherbranch (mini., fig. 6, PL 11) runs out laterally until it lies beneath

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the skin, to which it gives off a few fibres. As it proceeds for-wards it gradually becomes more ventral in position and subse-quently enters the ventral surface of the intermandibularmuscle, after having supplied the gular series of sensory organs.

Prom the foregoing it is evident that in Xenopus the ramushyomandibularis VII is composed of lateral line, motor, andgeneral cutaneous fibres, the homologue of the ramus mandi-bularis internus VII described and figured in Ban a by Strong(1895) being apparently unrepresented. The arrangement ofthe nerves at this stage is perhaps to be compared with thatobtaining in Si ren , in which Norris (1913) has observed anobvious division of the truncns hyomandibularis VII into ananterior lateral line ramus mentalis and a posterior ramusjugularis composed of motor and general cutaneous fibres. Thisseparation of the fibres is perhaps a little less obvious inX e n o p u s , but, judging by the distribution of the branches,there can be no doubt that the more ventral one (r.jug.) whichsupplies the orbito-hyoideus, depressor mandibulae, and ulti-mately the interhyoideus muscles, corresponds with the ramusjugularis of S i ren . The more anterior and dorsal nerve ishomologous with the ramus mentalis of Si ren, and of itsmain branches the one supplying the oral sensory organs(m.ext., fig. 6, PI. 11; fig. 28, PL 16) represents the ramus mentalisexternus, while the rather more postero-ventral branch (m.int.,fig. 6, PL 11; fig. 28, PL 16) ending in the intermandibular muscleis comparable with the ramus mentalis internus of Siren.In this connexion it may also be noted that Escher (1925) indescribing the sensory organs and their innervation in Anura ,although agreeing with Strong's interpretation of the composi-tion of the nerves, has referred to the anterior and posteriorparts of the ramus mandibularis externus of B a n a as therami mentalis externus and internus respectively, thus bringingthem more into fine with the Urode le condition.

The arrangement of the ramus hyomandibuiaris VII of meta-morphosing larvae and young frogs (fig. 18, PI. 12; fig. 29, PL 16)is perhaps more easily correlated with that of the larva ofB a n a . As in the larva the ramus hyomandibularis VII ofyoung Xenopus is found to be composed largely of lateral-line


fibres. Owing to the migration of the suspensorial region atmetamorphosis, it passes backwards for a short distance alongthe lateral wall of the auditory capsule, and is there joiued bythe communicating nerve from the IX—X complex. It is thendirected ventrally and is located mesially to the depressormandibulae muscle, its relation with this and other musclesbeing essentially similar to the larval nerve. It divides into astout ramus jugularis to the subhyoideus (interhyoideus) muscleand a ramus mentalis, the distribution of which differs markedlyfrom that of the larva. After a short anterior course theramus mentalis gives off a branch which supplies the skin (cut,fig. 29, PI. 16). Almost immediately it divides into anterior andposterior nerves, the arrangement being similar to that of thelarva of Ran a as described and figured by Strong (1895).The anterior nerve of the adult Xenopus corresponds withthe ramus mentalis externus of the larva, while the longerposterior branch (m.int, fig. 29, PI. 16) is the ramus mentalisinternus of the larva. Both of them innervate the anterior andposterior ventral sensory organs, the motor fibres observed inthe ramus mentalis internus of the larva, not being observedin the young frog.

8. Ramus P a l a t i n u s V I I .In Xenopus the fibres comprising the ramus palatinus VII

become markedly ganglionated and form the ventral part of theganglion pro-oticum. In the larva the nervus palatinus VII, how-ever, after separating from the ramus hyomandibularis VII andramus ophthalmicus profundus V (fig. 11, PI. 11; fig. 12, PL 12;fig. 27 i, PL 15) just posterior to the subocular fenestra, is a verydelicate nerve. In the adult it is much thicker and passes directlydownwards through its ventral foramen. An indication haspreviously been given of the important relation between thecourse of the palatinus VII and the conformation of the subocularregion of the chondrocranium, and reference has been made tothe fact that, as there is no basal process, the palatinus VIIdoes not become enclosed in the subocular shelf. On leaving thechondrocranium it lies in a marked ventral groove together withthe ramus hyomandibularis VII and the ramus ophthalmicus

HEAD OF XENOPUS 215

profundus V. It passes below the chondrocranium and over thedorsal surface of the internal carotid artery (pal, fig. 27 b, PL 15),but when the latter courses mesially to give off the cerebralartery, the ramus palatinus YII is situated laterally to it.

The nerve pursues a more or less straight course anteriorly,giving off twigs to the dorsal wall of the pharynx and mouth. Itis not located near any of the masticatory muscles as is thesimilar nerve in Siren (Norris, 1913). As in this latter genus,the ramus palatinus VII divides into two main branches, one ofwhich is medial (jpal.v figs. 28, 29, PL 16) and the other lateral(pal.2) in position. The latter is located just beneath thesubocular fenestra in the larva, while the medial branch runsbelow the floor of the chondrocranium and supplies the anteriorpart of the mouth. In Siren both nerves communicate witha branch of the ramus ophthahnicus profundus V, but, as hasbeen mentioned in connexion with the distribution of the ramusophthahnicus profundus V, in Xenopus there is only onesuch anastomosis. In both larva and adult this fusion is effectedwith a branch of the lateral palatine nerve immediately behindthe internal naris. In Ran a Bender (1906) describes a similaranastomosis taking place between the ramus median's, however,and the ramus maxillaris superior V. In the sections of Xeno-pus larvae the medial branch was observed to terminate behindthe lateral, but in young frogs it was traced anteriorly to thefloor of the nasal capsule (jpal.^ fig. 4, PL 10), where some of itsfibres innervate the glandula intermaxillaris (g.i.m.). Possiblythere is also a further fusion of the ramus ophthahnicus pro-fundus V and ramus palatinus VII fibres in the vicinity of thisgland, for both nerves send off short branches which ramifyamong the gland. Such a contact would be comparable withthe anastomoses of the medial palatine nerve of the Sala-m a n d e r described by Francis (1934).

The other palatine branches observed by Norris in Sir en areapparently unrepresented in X e n o p u s , but this is not re-markable, for, as Francis (1934) shows, they are not alwayspresent even in Urode l a .

The palatine nerve seems to be one of the least variable ofthe Amphibian cranial nerves, and its distribution i n X e n o p u s

216 NKLLIE P. PATBESON

is quite typical of the phylum. It shows some resemblance tothat of E a n a (Strong, 1895; Gaupp, 1899), but its ramifica-tions are less complicated. It differs from that of P r o t JUS,investigated by Benedetti (1933), in having two and not onemain branch, but it shows a close approach to the arrangementobserved in Siren by Norris (1913).

Brief mention has already been made of the absence of com-munis fibres in the truncus hyomandibularis of X e n o p u s .Most investigators have recorded their presence in one of thebranches of this nerve. In Urode la they constitute the ramusalveolaris, which is generally identified with the ramus mandi-bularis internus of Anura (Strong, 1895). This latter authorand others such as Bender (1906) and Francis (1934) haveconsidered' the possible homologies of this nerve and have con-cluded that it corresponds with the chorda tympani of mammals.It is, therefore, rather remarkable that such a nerve, which isso universally found, should not have been observed in Xeno-pus . There is no accounting for this absence of communisfibres in the ramus hyomandibularis VII of X e n o p u s , butit is to be noted that, taking the nerves as a whole, and especiallythose of the V-VII complex, there is a very obvious segregationof the different types of fibres. Thus, in each of the nerves certainfibres predominate. The supra-orbital trunk is a derivative ofthe lateral line nerve; the ramus ophthalmicus profundus V iscomposed of general cutaneous fibres; the ramus mandibularisV is a mixture of cutaneous and motor fibres; and the ramushyomandibularis VII, which is largely composed of lateral linefibres, also contains motor and cutaneous elements. Thesepoints are perhaps of little significance, but the fact neverthelessremains that as far as can be ascertained from the present seriesof preparations of X e n o p u s , the ramus palatinus VII is theonly source of communis fibres in the nervus facialis.

Nervi Glossopharyngeus a d Vagus .The posterior lateralis, the glossopharyngeus, and vagus

emerge separately from the brain, about two-thirds of the wayalong the medulla oblongata. The first of these has beendescribed and figured by van der Horst (1934), who shows that

HEAD OF XENOPUS 217

it enters the brain by many separate bundles which therenponbreak up into separate fibres. It lies dorsally in about the samehorizontal plane as the lateralis anterior. It passes outwardsand ventrally, joining the glossopharyngeus before emergingfrom the chondrocranium. With these two nerves the vagusroots also unite, and the large bundle of fibres which resultspasses out through the wide foramen jugulare below the pos-terior border of the auditory capsule. Outside the cranium thefibres form a large ganglionie complex (g.v., figs. 27 a, c, PL 15;figs. 28, 29, PL 16) in which it is difficult to determine thevarious fibres.

The posterior lateralis fibres remain dorsal in position in theglossopharyngeal-vagus ganglion, and in both larva and adultare the first to separate from the ganglion. The posteriorlateral-line nerve divides into anterior and posterior branchesto supply the trunk series of sensory organs, the anterior ramiextending to the neuromasts posterior to those innervated bythe truncus infra-orbitalis. In both larva and adult the pos-terior branch divides into two main lateral-line trunks (figs.28, 29, PL 16), those in the larva being continued into the caudalregion. One small dorsally directed lateral-line nerve (r.temp.,figs. 28, 29, PL 16) is doubtless comparable with the supra-temporal nerve of Urodeles (Norris, 1913; Escher, 1925).

1. Nervus G los sopha ryngeus .After passing into the glossopharyngeus—vagus ganglion, the

fibres of IX are difficult to distinguish from those of X. Theyare somewhat finer, and separate from the vagus shortly afterthe lateralis posterior fibres have been given off.

The nervus glossopharyngeus is directed anteriorly and issituated in a ventro-lateral groove of the auditory capsule belowthe crista parotica. It soon becomes ganglionated (g.gl., figs.27 a-c, PL 15; figs. 28, 29, PL 16), and from the ganglion theramus communicans to the ramus hyomandibularis VII (IXad VII, fig. 27 a, PL 15; figs. 28, 29, PL 16) separates dorsally.It takes a medial course and unites with the main stem of theramus hyomandibularis VII before the latter divides into therami jugularis and mentalis. This arrangement seems to be


typical of the A n u r a , but in the Urode la (Norris, 1913;Benedetti, 1933; Francis, 1934) the ramus communicans joinsthe ramus hyomandibularis VII very close to its dh ision into itsmain branches. In Urode la , therefore, the general cutaneousfibres pass almost directly into the ramus jugularis, but inAnura they mix with the other fibres composing the mainstem of the ramus hyomandibularis VII. As Strong (1895) hasshown in E a n a, the general cutaneous and motor fibres remainventral and pass into the ramus jugularis, while the ramusmentalis, although receiving some motor and cutaneous fibres,is usually largely composed of lateral line and communis fibres.

A short distance in front of the separation of the ramuscommunicans to the ramus hyomandibularis VII, the nervusglossopharyngeus in the larva divides into two main branches(IX, figs. 13, 14, PL 12; fig. 27 a, PL 15; fig. 28, PL 16) whichrun parallel for some distance anteriorly, and on reaching thethymus gland (th.gl., figs. 13,14, PL 12) are arranged one on eachside of it. They are directly comparable with the pre- and post-trematic branches of the glossopharyngeus of Urode la , buttheir homologies with the nerves of Anura are rather moreobscure. The dorsal one (jo.a.s., fig. 28, PL 16) resembles theramus pharyngeus of the larva of E a n a , but the ventralbranch (f.a.i., fig. 28, PL 16) shows some considerable differencesfrom the ramus lingualis IX, the course and composition ofwhich has been studied by Strong (1895). Bender (1906) dis-agrees with Gaupp's interpretation (1899) of the branches of thenervus glossopharyngeus in the adult E a n a , and recognizesa pharyngeus dorsalis IX, and a ramus lingualis, comparablewith the pharyngeus dorsalis and post-trematic nerves ofP i s c e s . Bender's findings are therefore in agreement withthose of Strong, and it seems likely that the two nerves presentin the larva of Xenopus are identical with those of E a n a .Some differences in the distribution of the ventral nerve are,of course, to be attributed to the absence of a tongue inX e n o p u s . Apart from this, it is also to be noted that there isless mixing of the fibres in the glossopharyngeus of X e n o p u sthan there is in E a n a , a feature also noticed in connexionwith the nervi trigeminus and facialis. Thus the dorsal or pre-

HEAD OF XENOPUS 219

trematic branch is composed of only communis fibres, agreeingin this respect with the ramus pharyngeus of E a n a , and thepost-trematic branch consists of motor fibres; in E a n a thelatter also contains general cutaneous and communis fibres.

The post-trematic branch (p.a.i., fig. 28, Pi. 16) pierces theventro-lateral process of the palatoquadrate (fig. 13, PI. 12), thuscoming to lie (p.a.i., fig. 12, PL 12) ventrally to the truncusinfra-orbitalis (io.), and with the constrictor branchialis musclebetween them. The nerve then extends antero-ventrally, lyingbetween the ventro-lateral process and the constrictor bran-chialis muscle, and passing through the fibres of the latterbecomes more ventral in position. It continues anteriorly be-yond the constrictor branchialis muscles and is located dorsallyto the cerato-hyale. More anteriorly it passes into a more medianposition (IX, fig. 7, PL 11) above the musculus interhyoideus,into the dorsal surface of which it eventually enters. On itsantero-ventral course this nerve also innervates the constrictorbranchiales I and II, and the musculi subarcuales recti I and II.

In the larva the dorsal branch of the nervus glossopharyngeuspasses gradually in a medial direction supplying the roof ofthe pharynx. It gives off a branch which applies itself to theventral surface of the ramus palatinus VII, but does not fusewith it. Its association with the palatinus is only of a temporarynature for it separates from it almost immediately and ends inthe roof of the mouth. Norris (1913) has reviewed the possiblecommunis anastomoses between the glossopharyngeus and facia-lis nerves in Urode la , and has shown that a Jacobson'sanastomosis between the ramus pre-trematicus IX and theramus palatinus VII is frequently established. A similar unionof these nerves has also been described in E a n a by Bender(1906). Owing to the fineness of the nerves in the larva ofX e n o p u s , it could not be determined with certainty if theassociation of these nerves should be regarded as a Jacobson'sanastomosis. There is certainly a very striking similarity be-tween the arrangement in Xenopus and that of such Uro-dela as Siren (Norris, 1913), P r o t e u s (Benedetti, 1933),and S a l a m a n d r a (Francis, 1934), but, as no communicationbetween the glossopharyngeus and facialis was observed in


young frogs, it seems doubtful if the larval connexion can beinterpreted as a Jacobson's anastomosis.

At metamorphosis the nervus glossopharyngeus is subjectedto certain changes. The post-trematic branch (p.a.i., fig. 29,PI. 16) becomes reduced and passes over the thymus gland andcrista parotica, ending in a few muscle-fibres, which are probablythe remains of the musculus constrictor branchialis II. In theyoung frog it could not be found, and the musculus inter-hyoideus (= subhyoideus) is innervated by the ramus jugularisVII alone. The one main ramus of the glossopharyngeus inthe young frog (IX, fig. 29, PI. 16) represents the pre-trematicusof the larva, but its distribution is somewhat different. It isdirected antero-ventrally, passing mesially to the cornu of thehyoid, and on reaching the lateral wall of the pharynx (IX,figs. 18 a-c, PL 12) it divides into three main branches. The dis-tribution of these nerves seems to vary; in some specimens allthree branches pass to the ventral wall of the pharynx andmouth: none of them innervates the dorsal wall of the pharynx,so that the communis fibres to it are furnished by the ramuspalatinus VII only. In slightly older specimens, which are prob-ably more representative of the adult condition, two of thethree rami run to the dorsal wall of the pharynx, while the re-maining branch is distributed ventrally.

2. Nervus Vagus .In both larval and adult Xenopus the roots of the vagus

are quite distinct from those of the lateralis posterior and thenervus glossopharyngeus. It was observed that in the larva thevagus arose by a series of seven roots passing out laterally fromthe medulla oblongata. The roots are obviously arranged intothree pairs, with the dorso-lateral root much more conspicuousthan the finer ventral root. The fourth dorsal root has no ventralcounterpart. In young frogs there are only three dorsal and twoventral roots, the second pair of the larva, which is thinner thanthe others, having probably fused with the third. As in thelarva there is no ventral root opposite the last dorsal root. Thepresence of these roots is of some significance, for it involvesa consideration of the controversial question of the segmentation

HEAD OP XENOPUS 221

of the head. Kingsbury (1926), who has reviewed the wholeproblem, maintains that serial repetition of the branchial archesdoes not coincide with the somatic metamerism, and that thefifth to the tenth cranial nerves are not to be regarded as seg-mental nerves. None of the larval stages of Xenopus hereindescribed was young enough to make a comparative study ofthe somites and cranial nerves possible, so that no attempt hasbeen made to study the possible metamerism in X e n o p u s .There is, however, an obvious similarity between the arrange-ment of the nerve roots and those described by Goodrich (1918)and de Beer (1922) in Blasmobranchs. Both of these authorssubscribe to the view of a segmentally arranged occipital regionof the skull, de Beer concluding that in Squalus there arenine somites. In this connexion these authors show that thevagus nerve possesses four dorsal roots and that the ventralroots in the four vagus somites are drawn from the nervushypoglossus, which in Blasmobranchs usually has no dorsalroot. In Squa lus de Beer (1922) has observed three rootsbelonging to somites 7-9 and states that 'since the eighthsomite is the last of the vagus segments, the 9th is morphologi-cally the 1st spinal or post-vagal' (p. 467). While it is beyondthe scope of the present discussion to expatiate on the acceptanceor otherwise of the theory of a segmentally arranged head, itis felt that some significance attaches to the similarity of thenerve roots in Xenopus and Elasmobranchs. This is furtherstrengthened by the presence in young frogs of a connexionbetween the vagus and the hypoglossus. Tensen (1927) hasobserved that in Pip a the nervus hypoglossus has no dorsalroot, but that there are two ventral roots. In Xenopus thehypoglossus arises similarly to P ip a in both larval and adultstages, the anterior ventral root being considerably weaker thanthe posterior. In tracing the distribution of the vagus in sectionsof a young frog, one branch was observed to communicate withthe ganglion of the hypoglossus nerve. This connexion was notobserved in the larva, not even during metamorphosis. It seemsto develop after metamorphosis, and was too conspicuous tobe in the nature of an individual variation. Strong (1895)remarks on a similar communicating branch between the vagus


and hypoglossus nerves in E a n a . Following Addens's (1933)observations it seems to obtain in many of the lower chordates,and it is the general consensus of opinion that the hypoglo, susmay supply some of the motor elements to the vagus nerve,probably those to the last ventral root.

The nervus vagus is a stout nerve, its distribution in larvaand adult differing mainly in connexion with the branchialsupply in the former. The branchial nerve (br.X., fig. 27 c,PI. 15; fig. 28, PL 16) also gives off a branch which passesoutwards through the commissura branchio-cranialis and thenruns ventrally, innervating the subarcuales recti III and IV, thetransversus ventralis II, and also the constrictor branchiales IIIand IV. The visceral branch (vis., figs. 28, 29, PL 16) divides, asin other Amphibia, into branches to the alimentary tract (gas.,fig. 29, PL 16), the lungs and the heart (cd., fig. 29, PL 16). Therecurrent laryngeal nerve (rln., fig. 29, PL 16) is directed forwardsand divides into branches which end in the laryngeal muscles. Inaddition to these, certain motor-nerves are observed in the adultto pass off anteriorly to the musculus cucullaris (mus.v fig. 29,PL 16), to the petrohyoideus (mus.2), and to the muscles and alsothe skin in the region just anterior to the shoulder (mus.3). Themotor nerve to the musculus cucullaris is generally interpretedas the nervus accessorius. Addens (1933) has made a carefulinvestigation of this nerve, and has found that in Gas t e ros -t e u s it arises from the beginning of the spinal motor columnand not from the caudal end of the vagus column. The musculustrapezius (= cucullaris) is, therefore, innervated by the XInerve, a fact that naturally is rather at variance with theassumption that this muscle is a derivative of one of the bran-chial muscles of the larva. Addens refers to an article by Vdlker(1908), who has shown that in Larus r i d i b u n d u s 'thetrapezius is formed by the fusion of split-off portions of theoccipital and first cervical myotomes' (p. 342). Addens, there-fore, supports the view that the trapezius is not a branchialmuscle, and furthermore indicates that the nervus accessoriusis derived from the first spinal nerve. The latter point is,perhaps, not always so clearly demonstrable as it is in G a s t e r -oSteus , but apart from this, as has been previously indicated,

HEAD OF XBNOPUS

it is the generally accepted opinion that some of the motorfibres in the vagus may originate in the anterior spinal nerve.In lower chordates the first spinal nerve is usually termed thehypoglossus; it may have no dorsal root, so that the secondspinal is the first complete nerve of the spinal column. Thenervus accessorius in most cases is not separable from the vagusas a distinct nerve. It seems to have merged with the vagusand merely appears as a motor branch passing off anteriorlyfrom the main visceral branch of X.

The interpretation of the composition and distribution ofthe main cranial nerves ofXenopus described in the foregoingmay be summarized as follows:

Nervus T r igeminus .

1. E a m u s m a n d i b u l a r i s : motor and general cutaneousfibres.

(a) 1st ramulus (md.j): anastomoses with truncus infra-orbitalis; comparable with middle accessory tri-geminal branch of E a n a.

(b) 2nd ramulus (md.2): passes below truncus infra-orbitalis and supplies floor of orbit.

(c) 3rd ramulus (md.3): to skin, comparable with theramulus labialis of S i r e n .

(d) 4th ramulus (md.^): to tentacle in larva.(e) 5th ramulus (md.5): is the ramulus mandibularis

superior to the lower jaw.(f) 6th ramulus (md.6): is the ramulus mandibularis

inferior to the musculus intermandibularis.

2. E a m u s O p h t h a l m i c u s P r o f u n d u s : general cu-taneous fibres.

(a) Eamulus ciliaris inferior (OJJ.J: associates with IIIand enters eyeball.

(b) Eamulus ophthalmicus profundus minor (op.2): in-nervates the skin above and behind the eye, andassociates with IV.

(c) Eamus maxillaris (op.3): to upper jaw.


(d) Eamulus nasalis internus (op.^: (i) Lateral branch(int.nas.) passing into the plica obliqua. (ii)Medial branch (int.nas.) passing ventro-medialljthrough nasal capsule and ending in skin of snout.

(e) Eamulus communicans ad VII (op.s and op.J:anastomoses with lateral branch of ramus pala-tinus VII.

(/) Eamulus nasalis externus (op.e): to skin lateral toand in front of nasal region.

Nervus Facialis.1. Truncus supra-orbitalis (fe.): lateral line fibres.

(a) Eamus ophthalmicus superficialis (so.): to all thesupra-orbital sensory organs.

(b) Truncus infra-orbitalis (io.): to the infra-orbitalsensory organs.

2. E a m u s H y o m a n d i b u l a r i s Qiym.): lateral line, motorand general cutaneous fibres.

(a) Eamus jugularis (r.jug.): to orbito-hyoideus, quad-rato-hyoangularis, and interhyoideus muscles.

(b) Eamus mentalis (r.ment.):(i) Eamus mentalis internus (r.int): to skin,

gular sensory organs, and larval inter-mandibularis muscle,

(ii) Eamus mentalis externus (r.ext.): to oralseries of sensory organs,

(iii) Eamus cutaneus (cut): as a separate nervein the adult.

3. E a m u s P a l a t i n u s (pal.): communisfibres.(a) Eamus palatinus medialis (pal.j): to roof of mouth.(b) Eamus palatinus lateralis (paZ.2): establishes a con-

nexion with ramus ophthalmicus profundus Vand ends in roof of mouth.

Nervus Glossopharyngeus : communis and motor fibres.1. E a m u s communicans ad V I I (IX ad VII): ana-

stomoses with stem of ramus hyomandibularisVII.

HEAD OF XENOPUS 225

2. Eamus p re - t r ema t i cus (p.a.s.): supplies roof ofpharynx in larva, and dorsal and ventral walls ofpharynx and mouth in adult.

3. Eamus pos t - t r ema t i eus (p.a.i.): present in thelarva, innervating musculi constrictor branchialesI and II, subarcuales recti I and II and inter-hyoideus.

Nervus Vagus-Nervus Accessorius: lateral line, com-munis, general cutaneous and motor fibres.

1. La t e r a l i s pos te r ior (gost.lat.): divides into anteriorand posterior branches to the trunk sensory organs,the former giving off a ramus supra-temporalis.

2. Eamus branch ia l i s (br.X): to branchial region andalso to musculi constrictor branchiales III andIV, and subarcuales recti III and IV.

3. Truncus in tes t ino-accessor ius (vis.):(a) Eamus accessorius (wms.j): to musculus eucullaris.(b) Eamus muscularis (mus.^): to musculus petro-

hyoideus.(c) Eamus muscularis (m«s.3): to skin and muscles of

shoulder.(d) Main Stem dividing into:

(i) Eamus laryngeus recurrens (rln.): passesforwards to laryngeal muscles and also totransversus ventralis IV.

(ii) Eamus intestinalis (gas.): to alimentary tract,also gives off a pulmonary branch.

(iii) Eamus cardis (cd.): to heart.

NO. 322


TABULAE SUMMARY OF COMPARISON OF DISTRIBUTION OF MOTORNERVES IN AMPHIBIA.

MUSCLE.

Levatores mandibulaeanterior and posterior

Orbito-hyoideus andquadrato-hyoangularis

Intermandibularis

Interhyoideus

Constrictores branchiales:I and II

III and IV

Subarcuales recti:I and II

III and IV

Transversus ventralis:I

IV

Cucullaris

Petrohyoideus

Geniohyoideus

Hyoglossus

Laryngeal muscles

INNERVATION.

Anura(Strong and

Gaupp)

R. mandibu-laris V

R. hyoman-dibularisVII

R. mandibu-laris V

R. hyoideusVII

••

••

R. acces-sorius

IX and X

Hypoglossus

Hypoglossus

II. laryn-geus

Urode la(Norris andFrancis)

R. mandibu-laris V

R. jugularisVII

R. inter-mandibu-laris V

R. jugularisVII

• •

R. recurrensX

R. acces-sorius

••

Hypoglossus

Hypoglossus

R. laryn-geus re-currens

X e n o p u s .

R. mandibularis V.

R. jugularis VII.

R. mandibularisinferior and R.mentalis inter-nus in larva.

R. jugularis VIIand R. post-tre-maticus IX inlarva.

R. post-trematicusIX.

R. branchialis X.

R. post-trematicusIX.

R. branchialis X.

R. branchialis X.R. laryngeus re-

currens X.

R. accessorius.

Truncus intestino-accessorius.

Hypoglossus.

Hypoglossus.

R. laryngeus re-currens.

HEAD OP XENOPUS 227

SUMMARY.

The foregoing account deals with some microscopic observa-tions on the main anatomical features of the head of X.1 a e v i s, in both larval and young adult stages.

The arrangement of the lateral line sensory organs of the larvahas been compared with that of the frog, and both are in generalagreement with the distribution of similar organs in the Uro-de la .

The larval musculature is found to be similar to that of X.f rase r i , described by Edgeworth. Notes regarding the changesoccurring at metamorphosis are given.

The course of the head vein and internal carotid artery hasbeen followed, as both blood-vessels are relevant to the studyof the chondrocranium and nerves.

A complete study of the chondrocranium in different stagesof larvae from the time of hatching up to metamorphosis revealsseveral points at variance with Kotthaus's findings. Meta-morphosis results in marked changes in the auditory and nasalregions. The former region has been studied by de Villiers inthe adult X e n o p u s , and is herein only briefly reviewed. Inregard to the nasal region, an account is given of the cavitiesand cartilages, both conforming in essentials to the typicalAnuran arrangement.

The hyobranchial skeleton of the larva and young frog arein close agreement with that of mature specimens investigatedby Eidewood.

The cranial nerves have been studied in some detail, and areobserved to depart in some respects from those of typicalA n u r a. The composition of the various nerves is in agreementwith that of both Anura and Urode la , but the arrangementof the nerves approaches that of such Urodela as Siren(Norris), P r o t e u s (Benedetti),and S a l a m a n d r a (Francis).This is perhaps most noticeable in the innervation of the upperjaw, for a maxillary nerve is not separated from the maxillo-mandibular stem as in most Anura ; the general cutaneoussupply to the maxillary region is derived in Xenopus fromthe ramus ophthalmicus profundus V as in some Urode la .

2 2 8 NELLIE F. PATEKSON

BEFERENCES.

Addens, J. L., 1933.—"The Motor Nuclei and Roots of the Cranial andKrst Spinal Nerves of Vertebrates", 'Zeitsehr. f. d. ges. Anat.', 1. Abt.,101.

de Beer, G. R., 1922.—"The Segmentation of the Head in Squalus acan-thias", 'Quart. Journ. Micr. Sci.\ 66.

1926.—"Studies on the Vertebrate Head. II. Orbito-temporalRegion of the Skull", ibid., 70.

—— 1937.—'Development of the Vertebrate Skull.' Clarendon Press,Oxford.

Benedetti, E., 1933.—"II Cervello e i Nervi Cranici del Proteus anguineus",'Mem. d. 1st. Ital. di Speleologia', Ser. Biol., Mem. III.

Bender, 0., 1906.—'Die Sehleimhautnerven des Facialis, Glossopharyngeusund Vagus.' Jena.

Bles, E. J., 1904.—"Life-history of Xenopus laevis Daud.", 'Trans. Roy.Soo. Edin.'j 41, pt. iii.

Broom, 11., 1903.—"Mammalian and Reptilian Vomerine Bones", 'Proe.Linn. Soc. N.S. Wales', pt. 4.

1935.—"The Vomer-Parasphenoid Question", 'Ann. Transv. Mus.',18, pt. 1.

Edgeworth, F. H., 1919.—"Development of the Hyobranchial andLaryngeal Muscles in Amphibia", 'Journ. Anat. London', 54.

1924.—"Autostylism of Dipnoi and Amphibia", ibid., 59.1929.—"Masticatory and Hyoid Muscles of Larvae of Xenopus

laevis", 'Journ. Anat. Camb.', 64.1935.—'Cranial Muscles of Vertebrates.' Camb. Univ. Press.

Escher, K., 1925.—"Das Verhalten der Seitenorgane der Wirbeltiere undihrer Nerven beim Ubergang zum Landleben", 'Acta Zool.', Arg. VI.

Eoske, H., 1934.—"Das Geruchsorgan von Xenopus laevis", 'Zeitsehr. f.Anat. u. Entwicklungsgesch.', 103.

Francis, E. T. B., 1934.—'Anatomy of the Salamander.' Clarendon Press,Oxford.

Gaupp, E., 1896-1904.—'Anatomie des Prosches', 1-3.Gilehrist, J. D. E., and von Bonde, C, 1919.—'Dissection of the Platanna

and the Frog.' Univ. of Cape Town.Goodrich, E. S., 1918.—"Development of the Segments of the Head of

Scyllium", 'Quart. Journ. Mior. Sci.', 63.1930.—'Studies on the Structure and Development of Vertebrates.'

Macmillan & Co., London.Grobbelaar, 0. S., 1924.—'Beitrage zu Einer anatomischen Monographic

der Xenopus laovia Daud.' Berlin,van der Horst, 0. J., 1934.—"Lateral Line Nerves of Xenopus", 'Psyohiat.

en Nourol. Blad.', no. 3 en 4.Kingsbury, B. F., 1926.--"Branchiomerism and the Theory of Head

Segmentation",' Journ. Morph. and Physiol.', 42, no. 1.

HEAD OF XENOPtfS 229

Kotthaus, A., 1933.—"Entwieklung des Primordial-Craniums vonXenopus laevis bis zur Metamorphose", 'Zeitgchr. f. wiss. Zool.', 144.

Muller, E., 1932.—"Unters. u. d. Mundhohlendriisen der Anuren Amphi-bien", 'Morph. Jahrb.', 70.

Norris, H. W., 1913.—"Cranial Nerves of Siren lacertina", 'Journ.Morph.', 24, no. 2.

Parker, W. K., 1876.—"Structure and Development of t i e Skull inBatrachia", 'Phil. Trans. Roy. Soe.', 166.

Peter, K., 1930.—"Development of the External Features of Xenopuslaevis", 'Journ. Linn. Soc. Zool.'

Ridewood, W. G., 1897.—"Structure and Development of the Hyo-branchial Skeleton and Larynx in Xenopus and Pipa; with Remarkson the Affinities of the Aglossa", 'Journ. Linn. Soc. Zool.', 26.

1900.—"Hyobranchial Skeleton and Larynx of a New AglossalToad, Hymenochirus Boettgeri", ibid., 27.

Rose, W., 1929.—'Veld and Vlei.' Speciality Press of South Africa,Capetown.

Schoonees, D. A., 1930.—"Skedelmorphologie van Bufo angostieeps(Smith)", 'S. Afric. Journ. Sci.', 27.

Strong, O. S., 1895.—"Cranial Nerves of Amphibia", ' Joum. Morph.', 10.Tensen, J., 1927.—"Einige Bemerkungen u. d. Nervensystem von Pipa

pipa", 'Acta Zool.', 8.du Toit, C. A., 1930, 1931.—"Skedelmorphologie van Heleophryne regis",

'S. Afric. Joum. Sci.', 27 and 28.G. P., 1933.—"Cranial Characters of Phrynobatrachus natalensis

(Smith)", ibid., 30.• G. P., and de Villiers, C. G. S., 1932.—"Die Skedelmorphologie van

Hyperolius horstoekii as Voorbeeld van die Polypedatidae ", ibid., 29.de Villiers, C. G. S., 1929.—"Development of a Species of Arthroleptella

from Jonkershoek, Stellenbosch", ibid., 26.1930.—"New Aspects of Anuran Osteology and Osteogeny", ibid., 27.1930.—"Cranial Characters of the South African Brevicipitid,

Phrynomerus bifasciatus", 'Quart. Journ. Micr. Sci.', 73.1931.—"Cranial Characters of the Brevicipitid Genus, Caeostemum

(Boulenger)", ibid., 74.1931.—"Further Notes on the Genus Cacosteroum including an

Account of the Cranial Anatomy of Caeostemum namaquense Werner",'S. Afric. Joum. Sci.', 28.

1931.—"Some Features of the Cranial Anatomy of Hemisus mar-moratus", 'Anat. Anz.', 71, no. 14/16.

• 1931.—"VS. d. Schadelbau der Brevicipitidengattung AnhydrophryneHewitt", ibid., 71, no. 14/16.

1931.—"U. d. Schadelbau des Breviceps fuscus", ibid., 72, no. 6/9.1932.—"VS. d. Gehorskelett der Aglossen Anuren", ibid., 74, no. 4/5.


De Villiers, C. G. 8., 1933.—"Breviceps and Probreviceps: Comparisonof the Cranial Osteology of two closely related Anuran Genera", ibid.,75, no. 12/14.

de Vos, C. M., 1935.—"Spelaeophryne and the Bearing of its CranialAnatomy on the Monophyletic Origin of the Ethiopian and MalagasyMiorohylids", ibid., 80, no. 13/16.

Winterhalter, W. P., 1931.—"Unters. ii. d. Stirnorgan der Anuren", 'ActaZoologica', 12.

EXPLANATION OF PLATES

All the illustrations were drawn with the aid of a Leitz drawingapparatus adjusted to the magnifications given. The followingtable is supplemental to that on p. 176, and summarizes thefigures illustrating the anatomical features of the various larval

Stage

Fig.

1

22 a

2

22 6

3

23

4

Notfigured

5

27

6

6-17, 24,28

7

1^,21 a,25,29

YoungFrog.

5, 18, 19,20,21 6.

EXPLANATION OF LETTERING.

etc., auditory capsule; aca., anterior cerebral artery; ace., accessory sensoryorgans; acus., acustic foramina; aim., anterior intermandibular muscleof young frog; ang., angulare; ary., arytenoid cartilage; as., auditory sacof young larva; asc.pr., ascending process of palatoquadrate; asm., anteriorsemicircular canal; at., annulus tympanicus; ha., basilar artery; bas.hy.,basihyale; bra., remains of branchial arches; br.pr., bronchial cartilage;car., internal carotid artery; cart.al., cartilago alaris; cart.obl., cartilagoobliqua; cart.obl.+pl., cartilago obliqua and planum terminate; cav.med.,cavum medium; cav.princ., cavum principale; c.br., musculi eonstric-tores branchiales; cer., cerato-hyale; cl. cloacal aperture; cnt., connectivetissue; con., musculus constrictor laryngis; cor., processus coronoideus;c.q.c.a., commissura quadrato-cranialis anterior; eric., cricoid cartilage;cris., crista intermedia; crp., crista parotica; csl., cloaeal sensory organs;c.tent., cartilage of tentacle; cm., cerebral vein; dat., dorsal part ofannulus tympanious; dep.man., musculus depressor mandibulae of larvae(quadrato-hyoangularis) j d.g.rn., ductus glandula nasalis medialis; dil.,museums dilatator laryngis; ec, ethmoidal cartilage; elf., endolymphaticforamen; elf.+acua., endolymphatic+acustic foramina; en., external nares;eus,, eustachian tube; exo., exoccipital condyle; fac, floor of auditorycapsule in young larva; /&., fenestra hypophyseos; / c , carotid foramen;f.jug., foramen jugulare; fl. fore-limb ;/o., fenestra ovale; f.m., foramenmagnum; fp., fronto-parietale; f.par., foramen parietale; f.pro., foramen

HEAD OF XBNOPUS 231

pro-oticum; gen., musculus geniohyoideus; g.i.m., glandula intermaxil-laris; gm., glandula nasalis medialis; Har., Harderian gland; Ac., hori-zontal cartilage from crista intermedia; hf. hyoglossal foramen; Id., hindlimb; hsc., horizontal semieular canal; hm., hyomandibular sensoryorgans; h.vn., head vein; hyog., museums hyoglossus; ilc., inferior labialcartilage; int.hy., musculus interhyoideus; int.man., museulus intermandi-bularis; iorb., infra-orbital sensory organs; Up., intertrabecular plate;lam.inf., lamina inferior; Ian., larynx; for., musculus laryngeus dorsalis;lev.ant., musculus levator mandibulae anterior; lev.hy., musculus levatorhyoideus ( = orbito-hyoideus); lev.post., musculus levator mandibulaeposterior; lev.man., musculus levator mandibulae in young frog; lev.tent.,museulus levator tentaculi; l.h.vn., vein from levator hyoideus; Lo.p.,larval otic process; max., maxillare; m.cu., musculus eucullaris; m.dep.man., musculus depressor mandibulae of young frog; me., middle ear;Meek., Meckel's cartilage; m.pt.hy., musculus petrohyoideus; m.ptg., medialpart of pterygoideus; msl., medial sensory organs; mus.pr., processusmuscularis of palatoquadrate; nas., nasale; n.l.d., duetus nasolaerimalis;not., notochord; o.e.c, os en ceinture (orbitosphenoid); ope, operculum;opha., ophthalmic artery; oph.vn., ophthalmic vein; osl., dorsal sensoryorgans; otp., otic process; par., parietal sensory organs; pea., posteriorcerebral artery; pch., parachordal; pip., pars interna plectri; pl.ani.,planum antorbitale; plee., plectrum; pis., pleurosphenoid; pl.term., pla,numterminate; pm., premaxillare; p.met., pila metoptica; pmp., pars media-plectri; porb., post-orbital sensory organs; ppc, post-palatine commissure;ppl., posterolateral process of hyobranchial skeleton; ppm., postero-median process of hyobranchial skeleton; p.ptg., processus pterygoideus;pro., pro-oticum; prq., paraquadratum; ps., parasphenoid; pt., plica ter-minale; ptg., pterygoideus; pt.vn., pituitary vein; quad., quadratum; r.e.,musculus rectus externus; rec.lat., recessus lateralis; rec.med., recessusmedians; rec.sac, recessus sacciformis; r.s., musculus rectus superior;sem., septomaxillare; sep., septum nasi; seth., supra-ethmoid; smp., sulcusmaxillopalatinus; s.o.b., subocular bar of palatoquadrate; sof., subocularfenestra; sorb., supra-orbital sensory organs; sp. 1 and 2, foramina for1st and 2nd spinal nerves; s.p.e., superior prenasal cartilage; spir., spira-culum; to., tectum anterius; tee., tectum nasi; tent., tentacle; th.f., thyroidforamen in hyobranchial skeleton; th.gl., thymus gland; thyr., thyroidgland; tp., tectum posterius; trab., trabecula cranii; utr., utriculus; vat.,ventral part of annulus tympanicus; vest., vestibulum; vlp., ventro-lateralprocess of palatoquadrate; vom., vomer (= praevomer); vsl., ventral sensoryorgans.

The main cranial nerves are indicated by Roman numerals (I to X);other abbreviations connected with the nervous system are as follows:

ant.lat., lateralis anterior; br.X, branchial nerve; cd., ramus cardis X ;cf., nerve to levator mandibulae posterior; cut., ramus cutaneus VH; d.al.,dorsal part of lateralis anterior; gas., ramus intestinalis X ; g.gl., ganglionglossopharyngeus; g.pro., ganglion pro-oticum; g.v., ganglion IX-X; hym.,truncus hyomandibularis; io., truneus infra-orbitalis; inf., infundibulum;int.nas., ramulus nasalis internus (op.4); md. 1-6, branches of ramusmandibularis V; m.ext., ramus mentalis externus VII; m.int., ramus


mentalis interims VII; mus.x, ramus accessorius; mus.2, nerve to petro-hyoideus muscle; mus.3, nerve to skin and shoulder; op., ramus ophthal-micus profundus V; p.a.i., ramus post-trematicus IX; pal., ramus palatinusVII; p.a.s., ramus pre-trematicus IX; pin., pineal body; pit, hypophysis;post.lat., lateralis posterior; r.jug., ramus jugularis VII; rln., ramuslaryngeus recurrens X ; r.man., ramus mandibularis V; r.merit., ramusmentalis VII; r.temp., ramus supratemporalis; so., ramus ophthalmicussuperflcialis; ts., truncus supra-orbitalis; v.al., ventral part of lateralisanterior; vis., truncus intestino-accessorius X.

PLATE 9.

Figs. 1-3.—Transverse sections through the nasal capsule of a tadpoletowards the end of metamorphosis. X 30.

PLATE 10.

Fig. 4. Transverse section through posterior region of nasal capsule ofa metamorphosing specimen. X 25.

Fig. 5.—Transverse section posterior region of nasal capsule of a youngfrog, passing through external opening of nasolacrimal duct. X 25.

PLATE 11.

Figs. 6 and 7.—Transverse sections through the olfactory region of atadpole measuring 60 mm. long, x 30.

Fig. 8.—Transverse section of tadpole showing the entrance of theramulus nasalis internus into the olfactory capsule. X 25.

Figs. 9 and 10.—Transverse sections through the telencephalon of a60 mm. long larva, showing the disposition of the nerves. In fig. 10the oculomotorius is dividing before encircling the ramus ophthalmicusprofundus V. X 25.

Fig. 11.—Transverse section through the oculomotor foramen in thetadpole. The ophthalmic artery can be seen passing out with the nerve.X25.

PLATE 12.

Fig. 12.—Transverse section through the carotid foramen of a larva.The artery is just dividing into two within the cranial cavity. X 25.

Figs. 13 and 14.—Transverse sections through the thalamencephalon oflarva, showing the separation of the nerve trunks from the ganglion pro-oticum. x 25.

Fig. 15.—Transverse section to show the exit of the cerebral vein fromthe cranial cavity on the right, and the junction of the pituitary andcerebral veins on the left. X 25.

Fig. 16.—Transverse section passing through the ganglion pro-oticumafter the separation of the truncus supra-orbitalis from it. x 25.

Fig! 17.—Transverse section passing through anterior region of cere-bellum before formation of the ganglion pro-otioum. X 25.

Figs. 18 a-c.—Transverse sections passing through the auditory capsuleof a young frog. The post-palatine commissure, part of the middle ear,and the plectral apparatus are visible. X 16-65. Fig. 18 a is 90/x in frontof Fig. 18 6, and there are 380/A between the latter and Fig. 18 c.

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HEAD OF XBHOPTJS 2 3 3

PLATE 13.

Fig. 19 a. Transverse section of young frog, passing through the auditorycapsule near the end of the pars intema pfectri and just anterior to theoperculum. x 46-5.

Fig. 19 6. Transverse section of voting frog, 40/A behind fig. 19 a. Thissection passes through the operculum, the lower end of which has not fusedwith the wall of the auditory capsule. X46-5.

Figs. 20 a and b. Transverse sections through the larynx of a youngfrog, showing the laryngeal muscles. Fig. 20 a is 110/x anterior to Fig.20 6. Both X50.

Fig. 21 a. Diagrammatic reconstruction of the hyobranchial skeletonof a metamorphosing specimen. Ventral view, x 8-4.

Fig. 21 6. Diagrammatic reconstruction of the hyobranchial skeleton ofa young frog. Ventral view, x 8-4.

Figs. 22 a and 6. Diagrammatic reconstructions of the chondrocrania ofvery young larvae, measuring 5 mm. and 7 mm. long respectively. Thesmaller larva had only recently emerged and its mouth was still closed.Dorsal views, each x 50.

Fig. 23. Diagrammatic reconstruction of the chondrocranlum of a larvameasuring 10 mm. long. Dorsal view. X 25.

PLATE 14.

Fig. 24. Diagrammatic reconstruction of the ehondrocranium of a larvameasuring 60 mm. long. Dorsal view, x 8-4.

Fig. 25 a. Diagrammatic reconstruction of the cartilages in the nasalcapsule of a metamorphosing larva. Side view. X16-65.

Fig. 25 6. Diagrammatic reconstruction of the suspensorial region ofthe same larva as in Fig. 25 a. With the exception of the fronto-parietale,the ossifications have been omitted. Side view. x8-4.

PLATE 15.

Fig. 26 a. Dorso-lateral view of the skull of adult X e n o p u s l a e v i s .Xapprox. 2-5.

Fig.266.VentralviewofskullofadultXenopus l a e v i s . xapprox.2-5.Fig. 27 a. Dorsal view of a reconstruction of the ehondrocranium and

cranial nerves of a larva measuring 28 mm. long. On the right side thecartilages have been removed to expose the ganglia and roots of the nerves.X 16-65. In this and subsequent diagrams of the nerves no attempt hasbeen made to indicate the different kinds of fibres.

Fig. 27 b. Ventral view of the same reconstruction as in Fig. 27 a.Only the articular regions of the palatoquadrate are given in order toshow the relative positions of the blood-vessels, some of the cranial nerves,and the parts of the palatoquadrate. X 16-65.

Fig. 27 c. Side view of the same ehondrocranium. The nerves are seenissuing from their respective foramina. X16-65.

Fig. 27 d. Diagram representing an inner view of the right half of thesame ehondrocranium as in Figs. 27 ct-c. In this diagram the positions ofthe nerves and blood-vessels within the ehondrocranium are shown.X16-65.


PLATE 16.

Fig. 28. Side view of a diagrammatic reconstruction of the main cranialnerves in a larva measuring 60 mm. in length. X16-65.

Fig. 29. Diagrammatic reconstruction of the main cranial nerves in ametamorphosing specimen. Side view, x 16-65.

Fig. 30 a. Side view of tadpole, showing positions of lateral line sensoryorgans observed in young stages. 30 mm. in total length; 11 mm. tocloacal aperture; tentacle, 2-5 mm. long. X 4-5.

Fig. 30 b. Dorsal view of a metamorphosing larva to show the arrange-ment of the lateral line sensory organs. 46 mm. in total length; 15-5 mm.to cloacal aperture; tentacle, 3-0 mm. long. x 4 .

Documents

Department of Zoology, University of the Witwatersrand ...7, 8, PI. 11) and the commissura quadrato-cranialis anterior. The ventral muscles of the larval head are two broad trans-verse