SRGMENTATION AND DIFFERENTIATION OF CHROMOSOMES. 2 8 5

Transverse Segmentation and Internal Differen-tiation of Chromosomes.

By

W. E. Agar,Glasgow University.

With Plates 12 and 13.

THE material for this paper was partly the same as was usedfor my former paper on the spermatogenesis ofLepidos i reu ,partly Lepidos i ren larvas obtained during- the same expedi-tion, and partly larvae collected by Prof. Graham Kerr on hisprevious expedition to the Paraguayan Chaco.

SOMATIC MITOSES.

In my paper on the spermatogenesis of Lepidosiren, itwas shown that the uuivalents of the diakinesis of the firstmeiotic prophase develop a vei'y marked transverse constric-tion. When these uuivalents pair ( i .e . the second pairing asdescribed in the paper), the transversely constricted consti-tuents of each pair form together a perfectly typical " tetrad."As has now been found to be the case in so many forms, thefour segments of each tetrad are not distributed to the fourgametes, but both divisions are longitudinal—that is, thetransversely constricted chromosomes (" dyads ") of anaphaseI split longitudinally to form tetrads again, and anaphase IIseparates chromosomes still transversely constricted as theywere in prophase I. These transverse constrictions are left,therefore, without any assignable significance. Attention wasdrawn in the paper alluded to to the probability that thetransverse.constrictions correspond with the apices of the VJs

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of tlie somatic or spermatogonial mitoses. Further evidencewill now be produced in favour of this conclusion, and to showthat this transverse segmentation is potentially present in allthe chromosomes in all parts of the body, though it is mostmarked whenever the chromosomes are particularly short,fn any larva of Lepi dosiren occasional mitoses are foundwith unusually short chromosomes, and these were found tobe extraordinarily numerous in one larva of Graham Kerr'sstage 31 + . They are present in probably all the tissues, butespecially in the nervous system.

Fig. 1 shows a metapliase in a nerve-cell from this larva.It is cut in three sections. It will be seen that the chromo-somes are very short, and that the majority of them aremarkedly transversely segmented. As they are all completelylongitudinally split also, the result is to form tetrads verysimilar to those found in meiosis, but present of course in thefull somatic number (thirty-eight). The scattering' of thechromosomes through the cell at this late stage is to be noted,and 1 believe that it is a frequent, though by no meansinvariable, characteristic of this type of mitosis that noequatorial plate is formed.

Figs. 2 and 3 are small fragments of two metaphases, alsoin nerve-cells, showing a few tetrads produced in the sameway as those in Fig. 1.

In all these figures it is noticeable th:it certain of thechromosomes are divided by their transverse joints into veryunequal portions.

Such, figures as these could be multiplied indefinitely fromthe same larva, and from others.

The question now arises, at what stage does the transverse[segmentation appear? As a rule, it seems not to do so tillthe metuphase, though there are numerous exceptions to this,as can be s-een in certain chromosomes in fig'. 8. Fig. 4 is apropliase of a somatic mitosis from the same larva as thatfrom which figs. 1, 2 and 8 are taken. This prophase wouldundoubtedly have resulted in very short, and hence trans-versely constricted metapliase chromosomes, but nevertheless,no certain segmentation is yet visible in them.

SEGMENTATION" AND DIFFERENTIATION OF CHROMOSOMES. 287

Fig. 5 shows an ana phase, such as would presumably resultfrom a metaphase like fig'. 1. The irregular scattering offche cliromosomes corresponds with the probable absence of adefinite equatorial plate mentioned above. There is no reasonto suppose that this irregularity of grouping leads to anunequal partition of the chromosomes to the daughter-nuclei.

Figs. 6 and 7 represent metaphase chromosomes fromsomatic mitoses with loug chromosomes. Fig. 6 is part of anucleus immediately before the chromosomes sire placed onthe spindle. Many of them are completely split into daughter-chromosomes, and some of them sj,iow little or no transversesegmentation. In certain of them, however (a, b, c, d), thisis very apparent, and it is to be noticed that the point cfsegmentation corresponds with the apex of the V. I t isstriking, too, that m some, especially d, the division occursmuch nearer one end than the other. It is true that in d thetwo short segments are a little affected by foreshortening,but this is not enough to account for more than a very smallparb of the difference in length, between the two limbs.

Fig. 7 is a later stage in so far that the equatorhil phite isfully formed, but nevertheless the chromosomes are not socompletely split as in fig. 6. The indication of transversesegmentation is so slight that the fact that it is representedby the apices of the V's could not have been recognisedwithout the help of the other figures.

A number of chromosomes from different mitoses in varioussomatic tissues of two larvaa are collected into fig. 8. Acomparison of these with the foregoing figures will suffice toshow the general nature of the segmentation in the somaticchromosomes.

The chief points to notice are:(1) The frequent very complete separation of the two

portions by the transverse joint. This separation often seemsat first sight to involve the whole chromosome, but closerinspection shows that it involves only the chromatin. How-ever separate the two chromatic portions may be, they arealways joined by a bridge of non-staining substance. This

288 W. E. AGAtt.

bridge may be as wide as the rest of the chromosome, or amere thread.

(2) The frequent inequality of the segments into which thechromosome is divided.

(3) The fact that the metaphnse split does not extendthrough the point of segmentation till after it is complete inthe other parts of the chromosome. This leads to the assump-tion by the chromosomes of various shapes. In the case oflong chromosomes it often leads to an appearance of twoV's connected by their apices by a thin strand (fig. 8, c, e, andfig. 6, a, b, c). Later, of course, the longitudinal split willextend through the connecting bridge also, and then an effectmay be produced as in n, fig. 8. More often, however, inthe case of long chromosomes the extension of the metapbasesplit through the connecting bridge is accompanied by a partialflattening out of the transverse constriction.

Another effect of the tardy splitting of the connecting-bridge is the frequent occurrence of X-shaped chromo-somes, some of which are shown in fig. 8 (g, h, j , h).Their mode of origin is obvious if compared with those chromo-somes in which the daughter halves of the two end-to-endsegments have not diverged so widely (fig. 8, e, and fig. 6,a, c).

SPEKJTATOGONIAL MITOSES..

In the newly formed spermatogonial equatorial plates thechromosomes attain a great length, and correlated with thisthere is little, if any, indication of transverse segmentation.Such an equatorial plate is shown in fig. 9. The same absenceof transverse segmentation, or even of sharp bends at theapices of the V's, is shown in fig. 6 of my former paper. Asthe chromosomes shorten, however, transverse segmentationbecomes evident in many of them, and at the time that themetaphase splitting takes place it is often very pronounced,especially in the smaller plates. These are very difficult toanalyse, as the confusion caused by the crowding together ofthe chromosomes far more than counter-balances the advantage

SEGMENTATION AND DIFFERENTIATION OF CHROMOSOMES. 289

gained by their smaller size. The best examples are to beobtained from the still further shortened chromosomes of thedaughter-plates. This is shown in fig. 7 of my former paper.1

The whole series of chromosomes from this nucleus is shownhere in fig. 15 A.

Figs. 10-12 are spermatogonial daughter-plates, and atransverse joint is very clear in many of the chromosomes,especially in the smaller ones.

Attention has already been drawn to the fact that thetransverse joint often divides the chromosome into two veryunequal segments, and also that the apices of the V's of thelonger chromosomes correspond with the transverse joints ofthe shorter ones. Special regard should be paid to figs. 10and 11 and 15 A in this respect. In all of these it is seen thatthe two longest chromosomes form V's with approximatelyequal limbs, while those next in size have very unequallimbs. As we follow down the series of chromosomes theV's gradually pass into dumb-bells in the shortest ones.

MEIOTIC PHASE.

The development and fate of the transverse constrictions ofthe rneiotic chromosomes was fully described in my formerpaper. Here it is only necessary to recall that there emergesfrom the synizeticmass the full number of long chromosomes,which become spaced out through the nucleus during dia-kinesis. At first ( i . e . while they are still long) they are nn-segmented, but as they shorten up the transverse constrictionsappear. Finally they become more or less dumb-bell or hour-glass shaped, and then pair to form the typical meiotictetrads.

It is unnecessary to describe the development of the trans-verse constrictions again here, so I have started with thefully formed tetrads. A complete series of these, from a cell

1 In the explanation of the figures, and on p. 26 of that paper, thisfigure is referred to as an equatorial plate. This is a slip, and it iscorrectly described as a daughter-plate on p. 22.

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in which the spindle is fully developed, but the chromosomesare not yet arranged on the equatorial plate, is given, infig. 15 B. Ametaphase, or early anaphase, of the first meioticdivision is shown in fig. 13. In d we see a tetrad ring,breaking simultaneously through both points of attachmentof the conjugants. In c we see the more usual condition,where one point of attachment has given way before theother, and the ring has straightened itself out. In a and ethe dissociation has gone further, the constituents of thetetrad remaining attached by a thin thread only.

The complete series of chromosomes from a nucleus at thesame stage is given in fig. 15 C.

Fig. 14 shows two daughter-plates resulting from a firstraeiotic division. It is somewhat unusual for daughter-platesto be formed in this way in the meiosis of Lep idos i ren , thechromosomes as a rule remaining bunched together near thetwo poles, while the spindles rotate for the second division.The longitudinal splitting preparatory for the second divisionis, therefore, exceptionally well shown in this figure. Eachchromosome again forms a tetrad, owing to the transverselyconstricted chromosomes of anaphase I having each dividedlengthwise.

The chromosomes forming the right-hand plate (some ofwhich are in the next section) are shown in fig. 15 d. Onlyeighteen tetrads were present, one having evidently beencarried away by the razor.

A consideration of the spermatogonial and meiotic figures atonce shows a very important fact, namely that the t r a n s v e r s esegmentat ion of a given chromosome always t akesplace at the same spot. In the case of the longer chromo-somes the segmentation is marked by the bend of the V, inthe shorter ones by a transverse coustriction. This constancyas to the point of bending or constriction can also be gatheredfrom the somatic mitoses, but partly owing to the greatercomplexity of the figures it is not possible to demonstrate itso clearly as can be done in the gonadic divisions.

In the four series of chromosomes shown in fig. 15 I have

SEGMENTATION AND DIFFERENTIATION' OP OHEOMOSOMES. 291

tried to arrange the chromosomes roughly in order. Thelarge pair, or large tetrad, can nearly always be easily recog-nised, but the gradation in size amongst the remainder is socontinuous, and disturbances due to foreshortening, etc., soinevitable, that it is impossible to attain to accuracy ofarrangement. Fortunately for our purpose, however, whilethe large pair of chromosomes always segments approximatelysymmetrically, at least the next four pairs in size segmentvery unequally. Hence, confining our attention to the fewlargest pairs only, we can see that among' them ah least theangles of the V's or transverse constrictions of the tetradsalways occur in the same region of any'given chromosome.While it is not possible to be sure of the identification of allthe other chromosomes, an examination of the figure leaveslittle doubt in one's mind that the same constancy holds for thesmaller chromosomes also. This impression is gathered stillmore strongly from the preparations themselves, in which itis easier to allow for irregularities due to foreshortening, etc.

The equality oE the segments of the pair of large chromo-somes and inequality of those next in size is shown incidentallyin figs. 24-81 of my former paper.

Another important fact brought out by a study of theroeiotic figures is that in the asymmetrical tetrads like endsare always applied to like. [That is, the small segment isapplied to small and large to large]. In the case of thesomatic tetrads this orientation follows from their mode offormation. The case of the meiotic tetrads is different. Asdescribed in my former paper, at the time that the transverseconstrictions are developed, the chromosomes may be lyingwidely scattered through the nucleus. Hence the juxta-position of like ends to like when they pair to form thedefinitive tetrads must be put down to an active cause.

LITERATURE.

Hany cytologists have noted the occasional occurrence of•" tetrads " or transversely segmented chromosomes in somatic

292 W. E. AGAK.

cells. Delia, Valle, in a paper in which he himself describeshalf-a-dozen nuclei with tetrads from various tissues of thesalamander, and from Bidder's organ in the toad, hascollected together a large number of references; Gregoirehas added to the list, and from these two authors it may beseen how widely distributed such occurrences are throughoutboth the animal and vegetable kingdoms. It is interestingto note that Gregoire and Wygaerts figure three transverselysegmented metaphase chromosomes of Tr i l l ium, in all ofwhich they are divided by the joint into two unequal lengths,and tlint on p. 15 they state that the commonest form ofanaphnse chromosome in this plant " est celle d'un V incom-plete forme d'une grande et d'une petite branche."

Besides della Valle, very few authors have publishedresearches directed specially to the occurrence of transversedivisions in somatic chromosomes. Popoff has describedthe appearance of tetrads in tlie liver-cells of P a l u d i n a ,generally in the full somatic number. These are closelysimilar to the tetrads appearing in the primary ob'cytes of thesame animal, and Popoff (like della Valle) considers that inboth cases they are due to a physiological abnormality ofthe cells in question.

An inquiry into somatic tetrads has been made by Hackerand his pupils. Hacker brought out the transverse segmen-tation of the chromosomes in developing Copepod eggs withdiagrammatic distinctness by the action of ether. This hasbeen done still more beautifully by Schiller, working onseveral species of Cyclops. His figures 7—15 show typicaltetrads, present of course (except in rare cases) in the somaticnumber.

Nemec has brought about the formation of tetrads insomatic plant tissues by the action of chloral hydrate.

This brief reference to the literature will be enough toshow that the tendency to transverse segmentation of chromo-somes is very widely distributed throughout the animal andvegetable kingdoms—probably, indeed, the potentiality tosuch segmentation is present in all chromosomes, and becomes

SEGMENTATION ANJ) DIFFERENTIATION OK CHROMOSOMES. 293

opei'ative especially often when they become, for any reason,unusually short in proportion to their length. In this casethe transverse constriction together with the metaphase splitgives the well-known tetrad appearance.

The only constructive theory as to the significance of thistendency to transverse segmentation that is known to me isHacker's theory of teleutosyndesis. It is unnecessary todwell long on this, however, as it seems very improbable thatit can be maintained in the face of the facts brought forwardin this paper. For, firstly, the theory requires the finalfusion of the end-to-end segments at some stage in the life-cycle to prevent the number of segments in each chromosomebeing doubled in each meiosis. I t is extremely improbablethat such a fusion ever takes place in L e p i d o s i r en , as the" doubleness •" is traceable in all the tissues of the animal,and is specially well marked in the daughter-plates of thespermatogonial divisions—that is, in the last divisions beforethe meiotic prophase where the end-to-end conjugation issupposed to take place again. Secondly, the constantinequality of the two segments of many chromosomes is verymuch against the theory that they have beeu formed by thepermanent pairing end-to-end of pairs of homologous ones.Thirdly, the theory is in any case of such a nature as couldonly be accepted on very strong direct evidence in itsfavour.

GENERAL CONSIDERATIONS AS TO THE NATURE AND SIGNIFI-

CANCE OP THE TRANSVERSE SEGMENTATION.

The immediate cause of the trausverse segmentation maywell be physical—perhaps the tendency may be for thechromatin to collect at the ends of the chromosomes and flowaway from the middle. This is possibly dependent on purelyphysical factors—perhaps electrical charges or surface ten-sion, both of which have often been supposed to play a largepart in the movements observable in mitosis. In accordancewith this view is the fact that the constrictions are best

294 W. E. AGAE.

marked when the chromosomes are shortest. Tliis has beenalluded to several times already, find for definite data I referto fig. 1 5 A, and to the description of the development of thetransverse constrictions in tlie shortening chromosomes ofthe meiotic prophase in my former paper.

If the chromosomes always segmented in tlie middle, or,failing that, at no definite spot, the matter would possessbut little interest. But the fact that the segmentation alwaysoccurs in the same chromosome at the same spot demon-strates that the chromosomes possess a cons tan tl eng thwi se d i f fe ren t ia t ion . For if a chromosome werehomogeneous, or if its internal differentiation were not con-stant, a transverse constriction developed by physical orother means would either always occur in the middle orelse at no constant spot.

The theoretical importance of inductive evidence of thisdifferentiation needs no emphasis. We learn, of course, verylittle about the nature of the differentiation. It may consistonly in a physical or chemical difference between the chro-matiti at the two ends of the chromosome; or it may be that,as demanded by theory, the differentiation lies in the chromo-ineres of which the prophase chromosome is composed, andthat when the tug comes tlie chromosome always gives waybetween that pair of chromomeres (always situated in the samespot in the chromosome) which are least firmly attached toone another. An indefinite number of other possibilitiespresent themselves, but all demand the hypothesis that a,given chromosome is always composed of the same differ-entiated portions arranged end-to-end in the same orderaloug its length. ,.

The fact that in the diakinetic pairing of asymmetricallydivided chromosomes to form the definite tetrads, like endsare always applied to like, is another indication of a constantdifferentiation, which is only made apparent by the probablyphysical factor, which produces the visible segmentation.Moreover it implies that there exists an attraction betweenlike parts of homologous chromosomes, not only between such

SEGMENTATION AND DIFFERENTIATION OF CHROMOSOMES. 295

chromosomes as a whole.1 The permanent forms of meioticchromosomes found in certain animals by Moore and Arnold,and by JValker, may reasonably be supposed to be due to thesame lengthwise differentiation.

Finally, the facts here described are plainly in favour ofthe theory of chromosome individuality. Quite recentlyMeves has attacked this theory, largely on the grounds thatchough size differences exist amongst the chromosomes of thesalamander, yet individual variation, and unavoidable distur-bances due to bending, foreshortening, etc., are so great as tomake impossible the constant identification of the samechromosome. He is also sceptical of: the possibility of arrang-ing the somatic chromosomes in pairs. Such negative evidenceas his cannot, however, be held to counterbalance positiveevidence gained from those forms in which the size and otherdifferences are great enough to appear through all disturbingfactors. This is certainly the case with, at any rate, the largepair of chromosomes in L e p i d o s i r e n .

SUMMARY. •

(1) The tendency for chromosomes to become transverselysegmented or constricted is a wide-spread characteristic. Itbecomes operative especially, but not solely, whenever thechromosomes are short in comparison with their length, ashappens normally in meiosis, and exceptionally iu somatictissues.

(2) The point at which the constriction or segmentationtakes place in any given chromosome is constant for thatchromosome, and is the same as the point at which it mostreadily bends to form the angle of the V when present inthat form.

1 Accepting the commonly held views as to chromosome individualityand reduction. Those who would hold the opinion that the scatteredchromosomes of the meiotic prophase are not whole chromosomes, butprecociously separated daughter halves, have still to explain the factthat when they come together again on the equatorial plate, like ends arealways applied to like.

296 W. E. AGAB.

(3) The constancy of the position at which transverse seg-mentation takes place indicates a constant differentiation ofthe chromosomes in a lengthwise direction. ^

(4) The presence of transverse constrictions or joints inchromosomes is not, without special evidence, to be taken astin indication of bivalency, or of a future division plane.

EXPLANATION OF CEHTAIN TERMS.

At the request of the Editor, I take this opportunity of remindingreaders that Wilson's ' The Cell in Development and Inheritance'contains an excellent glossary. The few distinctively cytological termsused above which are not to be found there or in the index of thatbook are given below.

Di a kinesis (V. Hacker, 1897).—The stage in the meiotic prophasefollowing synizesis, in which the chromosomes are scattered widelyapart in the nucleus.

Meiosis (J. B. Farmer and J. E. S. Moore, 1905).—The phase inwhich reduction of chromosomes takes place, including both maturationdivisions. It is convenient to designate the stages of the first andsecond divisions as metaphase I, metaphase II, etc. (Gregoire).

Somatic should be restricted to mitoses in the cells of the body out-side the germ-track. By some authors this term is applied to allmitoses outside the meiotic phase, including, therefore, those of thesperinato- and oo-gonia, but this misuse of the term is to be deprecated.

Synizesis (C. E. McClung, 1905).—The clumping together of thechromatin often observed in the meiotic prophase. I t was included inMoore's term, " synapsis."

Teleutosyndesis (V. Hacker, 1910).—A theory of chromosomeconjugation, according to which the conjugants are permanently unitedin the meiotic prophase.

LIST OF REFERENCES.

Agar, W. E.—"The Spermatogenesis of Lepidos i ren paradoxa,"' Quart. Journ. Micr. Sci.,' 57, 1911.

Delia Valle, P.—" Osservazioni di Tetradi in Cellule Somatiche," ' Attidella Reale Accad. d. Sci. Fis. e Mat. Napoli,' xiii, 1908.

Gregoire, V.—"Les Cineses de maturation dans les deux regnes (Secondmemoire)," ' La Cellule,' xxvi, 1910.et Wygaerts, A.—" Reconstitution du noyau et formation des

chromosomes dans les cineses somatiques," ' La Cellule,' xxi,1904.

SEGMENTATION AND DIFFERENTIATION OF CHROMOSOMES. 297

Hiicker, V.—"Mitosen im Gefolge aniitoseniihnlicher Vorgiinge,''' Anat. Anz.,' xvii, 1900.

" Brgebnisse imd Ausblicke in der Keimzellenforsclmng,"' Zeitschr. fiir Abstam. und "Vererbungslehre,' vii, 1912.

Meves, F.—" Ueber oligopyi-ene und apyrene Spermien mid iiber HireEntsfcehung, nacli Beobachtungen an Paludina und Pygsera,"' Archiv. fiir mikr. Anat.,' lxi, 1903." Chromosomenliingen bei Salamandra, nebst Bemerkungen zur

Individualitatstheorie der Chromosomen," ' Arch, fiir mikr.Anat./ lxxvii, 1911.

Moore, J. E. S., and Arnold, G.—"On the Existence of Permanent Formsamong Chromosomes of the First Meiotic Division in CertainAnimals," 'Proc. Roy. Soc.,' B, lxxvii, 1906.

Nemec, B.—' Das Problem der Befruclitmigsvorgiinge,' Berlin, 1910.Popoff, M.—"Eibildung bei Palndina vivipara und Chromidien bei

Paludina und Helix," 'Arch, fiir mikr. Anat.,' lxx, 1907." liber das Vorhandsein von Tetradenchromosomen in den

Leberzellen von P a l u d i n a vivipara," ' Biol. Centrlbl.,' xxviii,1908.

Schiller, I.—"liber kiinstliche Erzeugnng ' primitive!-' Kemteilungs-formen bei Cyclops," 'Archiv. fiir Entwicklungsmecli.,3 xxvii,1909.

Walker, C. E.—" On Variations in Chromosomes," ' Ai'ch. fiir Zellf.,'vi, 1911.

EXPLANATION OF PLATES 12 AND 13,Illustrating Dr. W. E. Agav's memoir on "Transverse Seg-

mentation and Internal Differentiation of Chromosomes."

[All figures were drawn with the Abbe camera, under a magnificationof 2500 (Zeiss 15 mm. Apochr., 12 comp. oc, drawing table at the levelof the microscope stage). In reproduction, all figures have been reducedto £ except fig. 15, which is reduced to §. Final magnification of figs.1-14, as reproduced, is therefore about 1875. All the figures are ofLepidos i ren paradoxa.]

PLATE 12.Fig. 1.—Metaphase, cut in three sections from a nerve-cell. Larva

of Graham Kerr's stage 31 +. Sublimate- acetic.Fig. 2.—Portion of a metaphase in a nerve-cell of the same larva.Fig. 3.—Portion of a metaphase in a nerve-cell of the same larva.

298 W. E. AGAK.

Fig. 4.—Portion of a late prophase from a mesenchynie cell in thesame larva.

Pig. 5.—Anaphase, nerve-cell of same larva.Fig. 6.—Chromosomes about to be placed on the spindle. Muscle-

cell, larva of stage 32 + . Sublimate-acetic.Fig. 7.—Polar view of equatorial plate from same larva. Only a few

of the chromosomes are shown.Fig. 8. —Chromosomes from various tissues of two lavvas (stages 31 and

31 -f). Sublimate-acetic.Fig. 9.—Polar view of an unsplit equatorial plate of a spermatogonium.

This is intact, in a 35 p celloidin section. Sublimate-acetic.Fig. 10.—Portion of anaphase of spermatogonial division. The two

large chromosomes are partially overlying one another. Sublimate -acetic.

Fig. 11.—Portion of late anaphase of spermatogonial division.Sublimate-acetic.

PLATE 13. •Fig. 12.—A pair of daughter-plates of a small spermatogonial division.

12 b has some chromosomes missing, 12 a is intact. The two plates werein the same 40 p celloidin section, mounted between two cover-slips, andthe figure was obtained by drawing the upper one from one side, thenturning the slide over and drawing the other one from the other side.The two large chromosomes are shown in 12 a, but one limb of one ofthem is very, much foreshortened. Sublimate-acetic.

Fig. 13.—Portion of metaphase, first meiotic division. The largetetrad is not shown. Fleniniing.

Fig. 14.-—Pair of daughter-plates from a first meiotic division. Notall the chromosomes are shown. Sublimate-acetic. 40 p celloidin.

Fig. 15.-—Entire series of chromosomes from four nuclei of differentstages : A. Spermatogonial daughter-plate, B. Tetrad rings just priorto formation of the equatorial plate of the first meiotic division, c.Metaphase, first meiotic division. D. Tetrads from daughter-plate offirst meiotic division (the right-hand plate of fig. 14.) The nucleus Ais intact in a 40 \i celloidin section. B, c and D are all cut in twosections, but probably in no case is any chromosome cut. In D one ofthe tetrads has evidently been canied away by the razor. In B and cthe tetrads are orientated as they would be if the spindle axis wereparallel with the top edge of the page. * Shape of chromosomeseriously affected by foreshortening, f Chromosome partially con-cealed by other chromosomes, so that accuracy is not certain. (Seefuller description in text, p. 291.)

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