Placental ontogeny of the Australian scincid lizardsNiveoscincus coventryiandPseudemoia spenceri

THE JOURNAL OF EXPERIMENTAL ZOOLOGY 282:535–559 (1998)

© 1998 WILEY-LISS, INC.

Placental Ontogeny of the Australian ScincidLizards Niveoscincus coventryi andPseudemoia spenceri

JAMES R. STEWART1* AND MICHAEL B. THOMPSON2

1Faculty of Biological Science, The University of Tulsa, Tulsa,Oklahoma 74104

2School of Biological Sciences and Wildlife Research Institute,University of Sydney, New South Wales 2006, Australia

ABSTRACT Models for the evolution of placentation among squamate reptiles have been stronglyinfluenced by early work on one lineage of Australian scincid lizards: one of three lineages thatinclude species with specialized allantoplacentation. The discovery of two types of complexallantoplacentae among species currently assigned to the Eugongylus species group led Weekes([1935] Proc. Zool. Soc. Lond., 2:625–645) to propose that a graded evolutionary sequence was ex-emplified by the morphological series of allantoplacental types that existed among Australian skinks.One of the model’s strengths is the incorporation of testable inferences of the relationship betweenplacental structure and function. However, descriptions of placental structure for some species areincomplete and subsequent taxonomic revisions have resulted in confusion concerning the speciesstudied. One of the ambiguities is the identity of Lygosoma weekesae (Kinghorn [1929] Proc. Linn.Soc. N.S.W., 54:32–33), one of two species for which the most specialized type of allantoplacentawas defined. The distinguishing characteristic of this placental type is the presence of a structureknown as a placentome. Subsequent to the original species description, Lygosoma weekesae wasnamed a synonym for Pseudemoia spenceri (Rawlinson [1974] Mem. Natn. Mus. Vict., 35:87–96),yet the placental description (Weekes [1929] Proc. Linn. Soc. N.S.W., 54:34–60) is thought to be ofNiveoscincus coventryi (Rawlinson [1975] Mem. Natn. Mus. Vic., 36:1–16). We describe placentalontogeny of N. coventryi and P. spenceri as a contribution to study of comparative placentationamong lygosomatine skinks. We conclude that the placental description for L. weekesae could nothave been N. coventryi, because a placentome is not a component of the allantoplacenta of N.coventryi. Further, the allantoplacenta of this species does not conform to previously defined cat-egories for Eugongylus group skinks. The allantoplacenta of P. spenceri contains a placentome thatis structurally congruous with the original description of placentation for L. weekesae (Weekes[1929] Proc. Linn. Soc. N.S.W., 54:34–60). Comparison of extraembryonic membrane and placentalstructure among the four viviparous and one oviparous species of Eugongylus group skinks forwhich data are available shows that each species exhibits characters that distinguish it from otherspecies, while additional characters are shared among all, or some, species. Based on a recentphylogenetic hypothesis, the distribution of allantoplacental types among these species does notsupport Weekes’ ([1935] Proc. Zool. Soc. Lond., 2:625–645) model of a graded sequence for theevolution of squamate placentation. J. Exp. Zool. 282:535�559, 1998. © 1998 Wiley-Liss, Inc.

There have been numerous evolutionary originsof viviparity among modern squamate reptiles(Blackburn, ’82, ’85; Shine, ’85) and placental trans-fer of nutrients (placentotrophy) occurs in all spe-cies that have been studied (Stewart, ’92; Blackburn,’92, ’95). Based on structural components, six pri-mary types of placentae occur (Stewart and Black-burn, ’88; Blackburn and Callard, ’97):

1. chorioplacenta2. bilaminar yolk sac placenta3. choriovitelline placenta

Abbreviations used: ALL, allantoic membrane; AM, amnion, BO,bilaminar omphalopleure; C, chorion; CA, chorioallantoic membrane;CV, choriovitelline membrane; EN, endoderm; G, gland; IVC, intra-vitelline cells; IYM, isolated yolk mass; SM, shell membrane; ST, si-nus terminalis; UT, uterus; YC, yolk cleft; YS, yolk sac.

Grant sponsors: Office of Research, University of Tulsa; Univer-sity of Sydney; and Australian Research Council.

*Correspondence to: J.R. Stewart, Department of Biological Sci-ences, Box 70703, East Tennessee State University, Johnson City,TN 37614. E-mail: [email protected]

4. chorioallantoic placenta5. omphaloplacenta6. omphalallantoic placenta

536 J.R. STEWART AND M.B. THOMPSON

This rich diversity, in number of independentevents and the structural composition of placentalorgans, is unparalleled among vertebrates. As a re-sult, squamate reptiles provide a unique opportu-nity to study parallel sequences in the evolution ofamniote fetal nutritional patterns. The first attemptto analyze placental diversity and reconstruct alikely sequence for the evolution of squamate pla-centation was that of Weekes (’35). Although Weekes(’35) speculated on the functional role of thechorioplacenta and yolk sac placenta (omphalo-placenta and omphalallantoic placenta) in the ini-tial stages of the evolution of placentotrophy, thepreeminent element of her model associates struc-tural diversity of allantoplacentae with level ofplacentotrophy. Based on three types of structur-ally distinct chorioallantoic placentae, Weekes (’35)constructed a morphological series. Functional char-acteristics were deduced for each of the structuraltypes and the resulting categories were incorporatedinto an evolutionary model. Weekes’ (’35) structuralcategories have provided the foundation for all sub-sequent work and this classification scheme hasbeen amended to include one additional type(Blackburn, ’93a) (Table 1).

Weekes’ (’35) analysis was based on study of 13species of Australian lygosomatine skinks (Harri-son and Weekes, ’25; Weekes, ’27a,b, ’29, ’30). Ofthese species, only five were found to have spe-cialized (i.e., type II or III) chorioallantoic placen-tae and all of these are currently classified in the

Eugongylus species group (Greer, ’74, ’79). TypeII placentation was discovered in three species col-lected from Tasmania (Weekes, ’30). These spe-cies, currently assigned to the genus Niveoscincus(Hutchinson et al., ’90), include N. ocellatus, N.metallicus, and, either N. pretiosus or N. micro-lepidotus, or both (Hutchinson et al., ’89). Recentstudy of placental ontogeny of N. metallicus con-firmed that this species has a specialized type ofchorioallantoic placenta but revealed the need toreevaluate characteristics of Weekes’ (’30, ’35) typeII placental category (Stewart and Thompson, ’94).

The most specialized allantoplacenta, type III,was found in Lygosoma (Leiolepisma) entrecas-teauxi (Harrison and Weekes, ’25; Weekes, ’29, ’30,’35) and L. weekesae (Weekes, ’29, ’35). Based oncurrent species distributions and a single museumspecimen, the descriptions assigned to L. entre-casteauxi most likely include two species currentlyclassified as Pseudemoia entrecasteauxii and P.pagenstecheri (Hutchinson and Donnellan, ’92;Thompson and Stewart, ’94). The identity of thespecimens from which the description of placen-tation of L. weekesae (Weekes, ’29) was derivedhas been uncertain because none of the specimenshave been recovered. The placental descriptionwas published as a companion paper to the origi-nal description of L. weekesae (Kinghorn, ’29).Rawlinson (’74) examined Kinghorn’s (’29) speci-mens and concluded that they should be assignedto P. spenceri, and noted further, that Weekes’ (’29)placental descriptions likely were derived fromanother species sympatric with P. spenceri. Thisspecies was described later as Leiolopisma coven-tryi (Rawlinson, ’75), presently Niveoscincuscoventryi (Hutchinson et al., ’90). Thus, specimensdescribed by Kinghorn (’29) are conspecific withP. spenceri, while those studied by Weekes (’29)to describe placentation are considered to be N.coventryi (Rawlinson, ’75). Because type III pla-centation is an uncommon and specialized condi-tion, the identity of species with this character iscritical to reconstruction of the pattern of placen-tal evolution among lygosomatine skinks.

A minimum of four extraembryonic membranesestablish contact with the uterus during develop-ment of viviparous squamates (Stewart andBlackburn, ’88) and regional differentiation ofsome of these membranes results in an evenhigher number of potential sites for maternal-fe-tal exchange (Yaron, ’85; Stewart, ’90; Blackburn,’93a). Thus, during uterine gestation, structurallydifferent types of transport epithelia vary tempo-rally, as well as spatially. A rigorous model for the

TABLE 1. Classification of squamate allantoplacentae

Type Description Source

I Close apposition of maternal and Weekes (’35)fetal vascular systems

II Uterine vessels on raised folds; Weekes (’35)chorionic ectoderm thickened

III Uterine wall adjacent to Weekes (’35)mesometrium consists of a seriesof vascularized folds lined withthick epithelium; chorionicectoderm thickened

Extensive interdigitation of fetal Blackburn (’93a)and maternal tissues, maternaland fetal epithelial cells withmicrovilli

IV Uterine wall adjacent to Blackburn (’93a)mesometrium consists of villousfolds with cuboidal/columnarepithelial cells that protrude intoan invagination of thechorioallantois; chorionicepithelium consists of two distinctcell types (giant binucleate andthin, columnar)

LIZARD PLACENTAL ONTOGENY 537

evolution of squamate placentation would considerthe structure and function of the entire interfacebetween the uterus and embryo. We studied pla-cental ontogeny of N. coventryi and P. spenceri toclarify the identity of species with Weekes’ (’35)type III allantoplacentae, but also to describe thedevelopmental sequence for comparison with otherEugongylus group skinks.

MATERIALS AND METHODSGravid female Niveoscincus coventryi (n = 21)

and Pseudemoia spenceri (n = 29) were collectedfrom Kanangra Boyd National Park, New SouthWales, in November and December of 1990, 1993and 1995. Females were returned to Universityof Sydney and either killed immediately or re-tained in captivity for a period of time. Femalesnout-vent length was recorded after death andfecundity was estimated from the number of ovi-ductal eggs. Uterine tissue and the enclosed em-bryos were removed surgically and fixed in eitherformol-dioxane-picric acid (Griffiths and Carter,’58), 10% formalin or 3% glutaraldehyde. Forma-lin and glutaraldehyde fixed material was usedfor staging of embryos according to Dufaure andHubert (’61). Material fixed in formol-dioxane-pi-cric acid was embedded in paraplast, sectionedat 7 µm and stained with hematoxylin and eosin.Twenty-eight embryos of N. coventryi (stages 30,32–38, 40) and thirty-four embryos of P. spenceri(stages 29–33, 35–40) were examined. Placentalterminology follows Stewart and Blackburn (’88).

RESULTSFemale size and fecundity

Gravid female P. spenceri had greater snout-ventlength (range = 52–62 mm; mean = 57.4 ± 2.3mm sd; n = 27) than N. coventryi (range = 35–46mm; mean = 41.8 ± 3.4 mm sd; n = 21) (P ≤ 0.001).However, litter size was greater for N. coventryi(range = 1–4; mean = 2.6 ± 0.7) compared to P.spenceri (range = 1–3; mean = 1.9 ± 0.5) (P ≤0.001). Litter size was correlated positively withfemale snout-vent length for both species (N.coventryi, litter size = –5.0 + 0.18 svl; P ≤ 0.001;P. spenceri, litter size = –2.9 + 0.08 svl; P ≤ 0.03).

Placental ontogeny ofNiveoscincus coventryi

Embryonic stage 30Four placental structures are present at this

embryonic stage: chorioplacenta, choriovitelline

placenta, chorioallantoic placenta and omphalo-placenta. The amnion closely surrounds the em-bryo and is separated from the chorion by anexocoelom immediately above the embryo (Fig. 1).The amnion does contact the chorion in some re-gions but the tissues do not appear to be adhered.Neither the chorion nor the amnion are vascu-larized. The chorion is composed of two cell lay-ers, an outer layer of low, cuboidal cells with large,prominent nuclei (ectoderm) and an inner layerof squamous cells (mesoderm). The uterus is vas-cularized in this region and the overlying epithe-lial cells are squamous. Contact between thechorion and uterus forms the chorioplacenta(sensu Stewart and Blackburn, ’88; Blackburnand Callard, ’97). The allantois contacts thechorion above the embryo, such that the chorio-allantoic membrane abuts the chorioplacentaalong one side (Fig. 1). Continued expansion ofthe allantoic vesicle will provide vascular supportfor the chorion as the chorioplacenta is overgrownby the chorioallantoic placenta. Cells of the chori-onic epithelium of the chorioallantoic placenta(Fig. 2) have a similar appearance to those of thechorioplacenta. The allantoic contribution to thechorioallantoic membrane is vascularized. Theuterine component of the chorioallantoic placentais also vascularized and a thin, squamous epi-thelium overlies the vessels. The choriovitellineplacenta, which is the predominant structure overthe surface of the yolk sac at the embryonic pole,extends from the chorioplacenta (Fig. 1) andchorioallantoic placenta (Fig. 3) approximately tothe equator of the egg. The chorioallantoic pla-centa advances at the expense of the chorio-vitelline placenta because the allantoic mesodermat the site of contact with the choriovitelline mem-brane grows as a wedge between layers of somaticand splanchnic mesoderm of the choriovitellinemembrane (trilaminar omphalopleure) (Fig. 3).Circulatory vessels are numerous in both thechoriovitelline membrane and uterus and theoverlying epithelial cells of both tissues are squa-mous (Fig. 4). Epithelial cells of the choriovitellinemembrane are smaller and contain smaller nucleithan the epithelium of either the chorioallantoicmembrane or the bilaminar omphalopleure. For-mation of the omphaloplacenta is incomplete. Abilaminar omphalopleure extends from the regionof the sinus terminalis at the equatorial marginof the choriovitelline membrane to enclose theabembryonic pole. An aggregation of intravitellinecells adjacent to the sinus terminalis delimits thecontact between the omphaloplacenta and chorio-


Fig. 1. Niveoscincus coventryi. Chorioplacenta of a stage30 embryo. ×1,000.

Fig. 2. Niveoscincus coventryi. Chorioallantoic placenta ofa stage 30 embryo. Allantoic blood vessels are indicated byarrowheads. The asterisk indicates the position of the allan-toic cavity. ×860.

Fig. 3. Niveoscincus coventryi. Embryonic stage 30. Con-

tact zone between the chorioallantoic placenta and chorio-vitelline placenta. The arrowhead indicates advancing cellsof the allantoic mesoderm. ×810.

Fig. 4. Niveoscincus coventryi. Choriovitelline placenta ofa stage 30 embryo. Arrowheads indicate onphalomesentericvessels in the yolk sac splanchnopleure. ×710.


vitelline placenta and extends as a continuous lin-ear array within the yolk mass parallel to thebilaminar omphalopleure (Fig. 5). These cells formthe inner boundary of the isolated yolk mass. To-ward the abembryonic pole, the intravitelline cellsform two thin strands separated by a cavity, theyolk cleft (Fig. 6). The yolk cleft parallels thebilaminar omphalopleure but does not extendacross the abembryonic region of the yolk sac. Theouter layer of epithelial cells of the bilaminaromphalopleure is cuboidal in shape immediatelybelow the sinus terminalis (Fig. 5), and is lowerin height near the abembryonic pole (Fig. 6).Large endodermal cells span the width of the iso-lated yolk mass, contacting both the ectodermalcells of the outer epithelium and intravitellinecells (mesoderm) that form the inner margin ofthe isolated yolk mass. The yolk sac splanchno-pleure lining the inner surface of the yolk cleft isnot vascularized because yolk sac vessels extendonly to the level of the sinus terminalis. A shellmembrane is not visible in any region about thecircumference of the egg.

Embryonic stage 32The allantoic vesicle is larger than in stage

30 embryos. As a result, the area of fusion be-tween the outer allantoic membrane and thechorion is greater and the chorioplacenta hasbeen supplanted because the chorionic contribu-tion to the chorioplacenta has been incorporatedinto a portion of the chorioallantoic membrane.The expanding allantoic vesicle has also reducedthe total area of the choriovitelline membrane.The chorioallantoic placenta is uniform in mor-phology over its entire surface and is similar tothat of stage 30 embryos. The choriovitelline pla-centa is also similar in structure to that of stage30 embryos with a rich vascular supply and thin,squamous epithelial cells. Development of theomphaloplacenta has progressed beyond stage 30embryos but this structure is still incomplete.Intravitelline cells extend below the sinus ter-minalis nearly to the abembryonic pole, but ayolk cleft is not present across the abembryonicmargin of the yolk sac. Vascularization of thesplanchnopleure is also in a formative stage andomphalomesenteric vessels extend only a shortdistance beyond the sinus terminalis.

Embryonic stage 34Expansion of the chorioallantoic membrane

reaches its maximal extent about the perimeterof the egg in stage 34 embryos. The chorioal-

lantoic membrane has supplanted the chorio-vitelline membrane and is in contact with thebilaminar omphalopleure (Fig. 7). This relation-ship persists throughout the remainder of de-velopment. The chorioallantoic membrane andbilaminar omphalopleure are separated by amass of cells that is associated with the mar-gin of the omphaloplacenta (Fig. 7). In contrastto earlier embryonic stages, epithelial cells ofthe chorioallantoic membrane are more attenu-ate and squamous in shape (Fig. 8). The chorio-allantoic membrane and uterus are thin andwell vascularized. The structure of the chorio-allantoic placenta is uniform over most of itssurface area. However, slight ridges overlyinguterine blood vessels are visible in some sec-tions at the juncture of the chorioallantoic pla-centa and omphaloplacenta (Fig. 7). Whenpresent, this uterine morphology is continuousacross the transition zone from one placentalstructure to the other. Formation of the ompha-loplacenta is complete as evidenced by an in-tact, continuous isolated yolk mass across theabembryonic pole. The outer epithelial layer ofthe bilaminar omphalopleure is composed of lowcuboidal cells (Fig. 9). The nearest vascular sup-port for the omphaloplacenta is contributed byvessels in the yolk sac splanchnopleure. Thesevessels are present over the entire abembryonicsurface of the yolk sac. The structure of theuterus of the omphaloplacenta is similar to thatof the chorioallantoic placenta, in that theuterus is well vascularized and has a squamousepithelium.

Embryonic stage 36The chorioallantoic membrane and adjacent

uterus over the embryonic pole of the egg arevery thin and well supplied with blood vessels(Fig. 10). Reduction in height of cells of the chori-onic epithelium, evident in stage 34 embryos, ismore pronounced. This morphology does differin a restricted area adjacent to the ompha-loplacenta on one side of a cross section of theegg (Fig. 11). For this region, the chorionic epi-thelial cells are larger and have an irregularshape and larger nuclei. These cells resemble theepithelial cells of the chorion of earlier embryos(Figs. 1, 2). The diameter of allantoic vessels isgreater in this region compared to the remain-der of the chorioallantoic membrane. Uterinevessels are likewise larger in diameter and, be-cause they are covered with a thin, squamousepithelium, they protrude into the uterine lu-


Fig. 5. Niveoscincus coventryi. Margin of the omphalo-placenta of a stage 30 embryo. ×960.

Fig. 6. Niveoscincus coventryi. Omphaloplacenta of a stage30 embryo. Arrowheads indicate intravitelline cells of the yolksac splanchnopleure. ×825.

Fig. 7. Niveoscincus coventryi. Contact zone between the

chorioallantoic placenta and omphaloplacenta of a stage 34embryo. A cluster of intravitelline cells (arrowhead) connectsthe bilaminar omphalopleure to the yolk sac splanchnopleure.The asterisk indicates the position of the allantoic cavity. ×625.

Fig. 8. Niveoscincus coventryi. Chorioallantoic placenta ofa stage 34 embryo. ×1,080.


Fig. 9. Niveoscincus coventryi. Omphaloplacenta of a stage34 embryo. The arrowhead indicates an omphalomesentericvessel in the yolk sac splanchnopleure. ×1,095.

Fig. 10. Niveoscincus coventryi. Chorioallantoic placentaof a stage 36 embryo. ×1,190.

Fig. 11. Niveoscincus coventryi. Embryonic stage 36. Zone

of contact between the chorioallantoic placenta and omphalo-placenta. Arrowheads indicate the multicellular partitionformed by intravitelline cells that separates the two placen-tal regions. ×1,255.

Fig. 12. Niveoscincus coventryi. Omphaloplacenta of astage 36 embryo. ×1,250.


men. The intravitelline cell mass, seen in asso-ciation with the sinus terminalis in stage 30 em-bryos, has developed into a partition, several celllayers in thickness, that connects the bilaminaromphalopleure to the yolk sac splanchnopleure.The isolated yolk mass of the omphaloplacentais continuous across the abembryonic pole of theegg (Fig. 12). Large endodermal cells extend fromthe ectodermal epithelium of the bilaminaromphalopleure to span the isolated yolk massand contact the intravitelline mesodermal cellson the inner surface. Epithelial cells of thebilaminar omphalopleure are cuboidal but varyin size regionally. Cells are taller at the mid-abembryonic pole and at the equatorial marginof the omphaloplacenta than in the interveningregions. The yolk sac splanchnopleure is wellvascularized but does not contact the epitheliumof the bilaminar omphalopleure because of theintervening isolated yolk mass. The uterine mor-phology (i.e., squamous epithelium and numer-ous blood vessels) appears no different thanearlier stages.

Embryonic stage 38The embryonic hemisphere of the egg contin-

ues to be separated from the abembryonic hemi-sphere by a cellular partition that extends fromthe bilaminar omphalopleure to the yolk sacsplanchnopleure (Fig. 13). This structure sepa-rates the yolk cleft from the exocoelom and iscomposed of a layer of mesoderm, which linesthe exocoelom, fused to a layer of intravitellinecells. The allantoic vesicle fills the exocoelom andabuts the partition. The chorioallantoic placenta,which covers the outer perimeter of the embry-onic hemisphere of the egg, occasionally exhib-its regional differentiation. The central region,which extends circumferentially from the me-sometrial region and includes most of the em-bryonic pole of the egg, is composed of a thinsquamous epithelium on both uterine and em-bryonic sides of the interface (Fig. 14). The al-lantois contains a dense array of blood vessels,as does the uterus. The morphology at the pe-rimeter of the chorioallantoic placenta differsslightly in some, but not all (e.g., Fig. 13), speci-mens, in that chorionic epithelial cells are largerand are supported by bigger allantoic vessels andthe uterus contains very low ridges overlyingblood vessels (Fig. 15). This morphology is ap-parent over only a small region of the perimeterof the chorioallantoic placenta adjacent to theomphaloplacenta and frequently on one side of

the cross section of the egg. The isolated yolkmass of the omphaloplacenta has regressed tothe extent that it has been resorbed in some re-gions, leaving blood vessels of the yolk sacsplanchnopleure closely apposed to the bilaminaromphalopleure (Fig. 16). Epithelial cells in theouter layer of the bilaminar omphalopleure arelow cuboidal in shape. The uterine eptheliumcontains squamous cells and is uniform in mor-phology at the abembryonic pole (Fig. 16). How-ever, uterine ridges, formed by a squamousepithelium overlying blood vessels, are presentat the margin of the omphaloplacenta adjacentto the chorioallantoic placenta (Fig. 15).

Embryonic stage 40The slight regional differentiation of the

chorioallantoic placenta that was characteristicof earlier embryonic stages is still present. Mostof the chorioallantoic placenta is characterizedby squamous uterine and chorionic epithelialcells (Fig. 17). Allantoic and uterine blood ves-sels are abundant. A small area at the marginof the omphaloplacenta, usually only on one sideof the cross section of the egg, differs (Fig. 18).Uterine blood vessels are larger in diameter andprotrude into the uterine lumen as low ridges.The epithelial cells are squamous. The chorionicepithelial cells are somewhat irregular in shapewith large nuclei. The egg is divided structurallyinto an embryonic and an abembryonic compart-ment by a thin cellular partition that extendsfrom the bilaminar omphalopleure to the yolksac splanchnopleure (Fig. 19). The isolated yolkmass of the omphaloplacenta appears as verythin scattered remnants in some specimens andis absent in others (Fig. 20). The epithelial cellsof the bilaminar omphalopleure are reduced inheight compared to stage 38 embryos. The yolksac splanchnopleure is vascularized and thesevessels are in close apposition to uterine vesselsbecause of the low height of the overlying epi-thelial cells. The uterine epithelium is squamousand vascularized.

Placental ontogeny ofPseudemoia spenceri

Embryonic stage 30Three placental structures are present: chorio-

vitelline placenta, chorioallantoic placenta andomphaloplacenta. The allantois extends acrossthe embryonic pole of the egg and contacts thechoriovitelline placenta at the margin of the yolk


Fig. 13. Niveoscincus coventryi. Embryonic stage 38. Con-tact between the chorioallantoic placenta and the omphalo-placenta. The asterisk marks the position of the allantoiccavity. The arrowhead indicates the partition between thetwo placenta regions. ×940.


Fig. 15. Niveoscincus coventryi. Embryonic stage 38. Zone

of contact between the chorioallantoic placenta and omphalo-placenta. This specimen exhibits regional differentiation ofthe chorioallantoic placenta (compare with Fig. 13). The as-terisk marks the position of the allantoic cavity. ×890.

Fig. 16. Niveoscincus coventryi. Omphaloplacenta of astage 38 embryo. Arrowheads indicate omphalomesenteric ves-sels in the yolk sac splanchnopleure. ×1,445.



Fig. 18. Niveoscincus coventryi. Chorioallantoic membraneadjacent to the omphaloplacenta of a stage 40 embryo. ×800.

Fig. 19. Niveoscincus coventryi. Zone of contact betweenthe chorioallantoic placenta and the omphaloplacenta of a

stage 40 embryo. The asterisk marks the position of the al-lantoic cavity. The arrowhead indicates the partition sepa-rating the two placental regions. ×820.

Fig. 20. Niveoscincus coventryi. Omphaloplacenta of astage 40 embryo. The arrowhead indicates the outer epithe-lium of the bilaminar omphalopleure. ×1,445.


sac lateral to the embryo. The uterine and chori-onic epithelial cells are squamous, or low cuboi-dal, throughout most of the surface of thechorioallantoic placenta (Fig. 21). An exceptionto this morphology is a region adjacent to the me-sometrium in which the uterine epithelium israised over blood vessels and has a ridged ap-pearance (Fig. 22). The uterine ridges are cov-ered by a cuboidal epithelium. The chorionicepithelial cells of this region are also cuboidal,with a width that exceeds their height. These cellsare larger and contain larger nuclei than the uter-ine epithelial cells. Allantoic vessels are numer-ous and in close contact with the chorion.

The choriovitelline placenta, which extendscircumferentially from the chorioallantoic pla-centa, covers the equatorial region of the egg (Fig.23). The uterine epithelium of this structure israised overlying blood vessels and the resultingridges are more extensive than those of the chorio-allantoic placenta. Uterine ridges overlap thechorioallantoic placenta and omphaloplacenta,which lie on either side of the choriovitelline pla-centa. Uterine epithelial cells are low cuboidal.Simple, unbranched glands occur in the laminapropria below the epithelium. The epithelial cellsof the choriovitelline membrane (trilaminaromphalopleure) are squamous and this structureis well vascularized.

A bilaminar omphalopleure extends from thechoriovitelline membrane to encircle the abem-bryonic pole of the egg. Contact between thechoriovitelline membrane and the bilaminaromphalopleure is demarcated by a small clusterof cells located at the abembryonic edge of thesinus terminalis (Fig. 24). The morphology of thebilaminar omphalopleure varies markedly indistinct regions of the abembryonic pole, presum-ably reflecting different stages in the develop-ment of the omphaloplacenta. A vascularizedyolk sac splanchnopleure is a prominent featureimmediately below the sinus terminalis. Intra-vitelline cells extend as a continuous doublestrand parallel to the bilaminar omphalopleurefrom the cell mass associated with the sinusterminalis toward the abembryonic pole. The in-ner layer of intravitelline cells contributes to theyolk sac splanchnopleure. Omphalomesentericvessels do not reach the abembryonic pole. Thereis a prominent concentration of vessels at theleading edge of the vascularized region of spl-anchnopleure (Fig. 25). Yolk is not visible be-tween the outer layer of intravitelline cells andthe bilaminar omphalopleure in the region

bounded by the choriovitelline membrane andthe abembryonic extension of omphalomesentericvessels, but large irregular-shaped cells withlarge nuclei are common (Fig. 26). In contrastto the squamous epithelium of the choriovitellinemembrane, the outer epithelium of the bilaminaromphalopleure is cuboidal immediately below thesinus terminalis (Fig. 24), but this epitheliumis columnar over most of the area adjacent tothe vascularized region of the splanchnopleure(Fig. 25). The columnar epithelium extends somedistance beyond the vascularized region, as well(Fig. 26).

An isolated yolk mass, with very small yolkgranules compared to the yolk sac proper, ex-tends from the region of vascularized splanch-nopleure across the abembryonic pole (Fig 27).The splanchnopleure is not vascularized at themid-abembryonic pole and the outer layer of thebilaminar omphalopleure consists of thin, squa-mous cells in this region. Large irregular-shapedcells are visible occasionally within the isolatedyolk mass.

The uterine epithelium is columnar through-out the omphaloplacenta and contains distinctridges in regions apposed to the vascularizedsplanchnopleure (Figs. 24–26). The ridges con-sist of extensions of the lamina propria and theassociated epithelial cells and are well vascular-ized. Simple unbranched glands are sparsely dis-tributed at the base of the ridges (Figs. 25–26).A shell membrane is visible about the perimeterof the egg, but this structure is neither uniformin thickness, nor evenly distributed. It is thinand disjunct over the embryonic pole of the eggand thick, continuous, and folded in some regionsover the abembryonic pole (Fig. 27).

Embryonic stage 33The choriovitelline placenta, chorioallantoic pla-

centa and omphaloplacenta are all prominent.The chorioallantoic placenta contains three re-gions that exhibit distinctly different morpholo-gies. The placentome, along the mesometrialregion of the embryonic pole, is more elaboratecompared to stage 30 embryos (Fig. 28). Chori-onic epithelial cells are enlarged and cuboidal inshape with a height that is slightly greater thantheir width. Allantoic vessels are more numerous.The ridges of the uterine epithelium are deeperand more complex and uterine blood vessels arelarger and more extensive. The uterus of thechorioallantoic placenta surrounding the pla-centome is not ridged and both the uterine and


Fig. 21. Pseudemoia spenceri. Chorioallantoic placenta ofa stage 30 embryo. The asterisk marks the location of theallantoic cavity. ×1,120.

Fig. 22. Pseudemoia spenceri. Chorioallantoic placentomeof a stage 30 embryo. The asterisk indicates the position ofthe allantoic cavity. ×1,400.

Fig. 23. Pseudemoia spenceri. Embryonic stage 30. Zoneof contact between the choriovitelline placenta and the chorio-

allantoic placenta. The asterisk indicates the position of theallantoic cavity. ×760.

Fig. 24. Pseudemoia spenceri. Embryonic stage 30. Zoneof contact between the choriovitelline placenta and theomphaloplacenta. The arrowhead marks the mass of intravi-telline cells in contact with the sinus terminalis. Arrows in-dicate an array of intravitelline cells extending parallel tothe bilaminar omphalopleure. ×945.


Fig. 25. Pseudemoia spenceri. Embryonic stage 30. Ompha-loplacenta in the region of farthest advance of splanchnopleu-ric vessels toward the abembryonic pole. Arrowheads markomphalomesenteric vessels in the splanchnopleure. ×1,000.

Fig. 26. Pseudemoia spenceri. Embryonic stage 30. Om-phaloplacenta in a region intermediate between that shownin Figs. 25 and 27. ×1,175.

Fig. 27. Pseudemoia spenceri. Omphaloplacenta beyondthe advance of splanchnopleuric vessels in a stage 30 em-bryo. ×1,490.

Fig. 28. Pseudemoia spenceri. Chorioallantoic placentomeof a stage 33 embryo. The asterisk indicates the position ofthe allantoic cavity. ×860.


Fig. 29. Pseudemoia spenceri. Choriovitelline placenta ofa stage 33 embryo. The asterisk marks the location of theallantoic cavity. ×445.

Fig. 30. Pseudemoia spenceri. Choriovitelline placenta ofa stage 33 embryo. The asterisk indicates the position of theallantoic cavity. Arrowheads indicate uterine glands. ×1,200.

Fig. 31. Pseudemoia spenceri. Omphaloplacenta of a stage

33 embryo. Arrowheads indicate omphalomesenteric vesselsin the yolk sac splanchnopleure. ×1175.

Fig. 32. Pseudemoia spenceri. Contact between the chorio-allantoic placenta and omphaloplacenta of a stage 35 embryo.The arrowhead indicates the mass of intravitelline cells thatcontributes to the partition between the two placentae. Theasterisk marks the position of the allantoic cavity. ×700.


chorionic epithelia are squamous. The lowheight of epithelial cells places the numerousblood vessels in close proximity. The third re-gion of chorioallantoic placental differentiationis a circumferential band adjacent to the chorio-vitelline placenta (Fig. 29).

The choriovitelline placenta persists as a nar-row band at the equator of the egg between thechorioallantoic placenta and omphaloplacenta(Fig. 29). Although the total surface area ofchoriovitelline placentation is reduced comparedto stage 30 embryos, the distinctive morphologyof this structure is not diminished (Fig. 30). In-deed, the folds or ridges of the uterine epithe-lium are more prominent. The ridges are coveredwith columnar epithelial cells and are interwo-ven with blood vessels. The lamina propria of theuterus is thick and contains simple, unbranchedglands. The epithelium of the choriovitellinemembrane is squamous and blood vessels lie inclose support. The uterine folds extend a shortdistance onto both the adjacent chorioallantoicplacenta and omphaloplacenta, although theyreach their greatest height where they contrib-ute to the choriovitelline placenta (Fig. 29). Thatportion of the chorioallantoic placenta adjacentto the choriovitelline placenta superficially re-sembles the choriovitelline placenta. The heightof uterine ridges is less pronounced, but uterinefeatures are otherwise identical. Like the chorio-vitelline membrane, the epithelium of the chorio-allantoic membrane is squamous and allantoicblood vessels lie near the uterine lumen. Al-though the two extraembryonic membranes aresimilar morphologically, the circulatory provisionis different because the omphalomesenteric ves-sels supply the choriovitelline membrane, whileallantoic vessels provide circulation to the chorio-allantoic membrane.

The isolated yolk mass of the omphaloplacentais recognizable as sparsely distributed remnants,except at the central abembryonic pole where itpersists. The splanchnopleure is vascularizedabout the entire perimeter of the yolk sac. Theresorption of the isolated yolk mass positions theyolk sac splanchnopleure adjacent to the bil-aminar omphalopleure (Fig. 31). The yolk cleftforms an anatomical space that intervenes be-tween the omphalomesenteric vessels and thebilaminar omphalopleure. The epithelium of thebilaminar omphalopleure is cuboidal. The uter-ine epithelium is columnar and forms ridges overblood vessels. Simple, unbranched tubular glandsare scattered at the base of the epithelium. A rem-

nant of the shell membrane and large mass ofmaterial is present in the uterine lumen at theabembryonic pole, but this material does not oc-cur in the uterine lumen over the embryonic hemi-sphere of the egg.

Embryonic stage 35The choriovitelline placenta is no longer present.

The choriovitelline membrane has been sup-planted by the allantois, which now abuts the ag-gregation of intravitelline cells at the equatorialmargin of the omphaloplacenta (Fig. 32). Thechorioallantoic placenta contains the same re-gional differentiation apparent in stage 33 em-bryos. The placentome and adjacent region ofsmooth uterine epithelium do not differ from stage33 embryos. The morphology of the third region,at the outer perimeter of the chorioallantoic pla-centa is the same as stage 33 embryos, exceptthat this area now is positioned adjacent to theomphaloplacenta.

The isolated yolk mass of the omphalopla-centa is entirely absent but other features ofthe omphaloplacenta do not differ from stage33 embryos.

Embryonic stage 36The alignment of extraembryonic membranes

achieved by embryonic stage 35 is still presentand will remain so until parturition. The allan-toic vesicle fills the embryonic hemisphere of theegg and the yolk sac occupies the abembryonichemisphere. The chorioallantoic placenta andomphaloplacenta continue to be separated by in-travitelline cells extending from the bilaminaromphalopleure to the yolk sac splanchnopleure(Fig. 33). Three divisions of chorioallantoic pla-centation are still evident, although the regionadjacent to the omphaloplacenta is not as welldefined because the uterine ridges are much re-duced (Fig. 33). The morphology of the placentomeof the chorioallantoic placenta differs little fromstage 35 embryos. The chorionic epithelium con-sists of large, cuboidal cells, some of which appearbinucleate and the allantois is well vascularized(Fig. 34). The uterine ridges surround an exten-sive network of blood vessels (Fig. 35). The re-gion of the chorioallantoic placenta adjacent tothe placentome has a squamous epithelium onboth uterine and chorionic faces.

The bilaminar omphalopleure of the omphalo-placenta continues to consist of an outer layer ofcuboidal cells. The height of these cells varies re-gionally and is relatively low adjacent to the


Fig. 33. Pseudemoia spenceri. Embryonic stage 36. Zoneof contact between the chorioallantoic placenta and omphalo-placenta. The asterisk indicates the position of the allantoiccavity. The arrowhead marks the partition between the twoplacental regions. ×735.

Fig. 34. Pseudemoia spenceri. Placentome of the chorioal-lantoic placenta of a stage 36 embryo. ×1135.

Fig. 35. Pseudemoia spenceri. Tangential section throughthe placentome of the chorioallantoic placenta of a stage 36embryo. Arrowheads indicate interconnecting vessels that con-tribute to the structure of the uterine ridges. ×735.

Fig. 36. Pseudemoia spenceri. Omphaloplacenta of a stage36 embryo. The arrowhead indicates an omphalomesentericvessel in the yolk sac splanchnopleure. ×1,110.


chorioallantoic placenta and at the central abe-mbryonic pole. Three layers of cells are visiblebetween the uterine epithelium and the endot-helial cells of the splanchnopleuric vessels (Fig.36). Columnar cells, which are present overmuch of the uterine surface of the omphalo-placenta, are most evident in the region whereembryonic epithelial cells are tallest. The uter-ine epithelium is formed into low ridges associ-ated with blood vessels in this region. Simpleglands occur in the uterus at the base of theridges. Material is present in the uterine lumenand it is most obvious at the mid-abembryonicregion. Some of this material represents rem-nants of the shell membrane.

Embryonic stage 38No changes are apparent in placental histology

of stage 38 embryos compared to stage 36 embryos.The regional differentiation of the chorioallantoicplacenta persists and the placentome (Fig. 37) isintact structurally. The omphaloplacenta likewiseretains all of the features characteristic of stage36 embryos (Fig. 38).

Embryonic stage 40All placental structures established by embry-

onic stage 35 are present in terminal stage em-bryos although details of histology differ. Therelative surface area of the placentome and theheight of the uterine folds appear to be reduced(Fig. 39). The vascular supply to the allantois anduterus appears to be as extensive as in previousembryonic stages. The region of uterine ridges ad-jacent to the omphaloplacenta is much reduced.The predominant morphology for the chorioallan-toic placenta is a smooth uterine epithelium con-sisting of squamous cells (Fig. 40). The apposingchorionic epithelium is also squamous.

The egg continues to be divided into two hemi-spheres by a partition (Fig. 41). The height ofuterine and fetal epithelial cells are reduced oversome of the surface of the omphaloplacenta, butmany regions do not differ from stage 38 embryos(Fig. 42). The yolk sac appears empty.

DISCUSSIONSynopsis of placentation of

Niveoscincus coventryiAs with other squamates (Stewart, ’85; Stewart

and Blackburn, ’88; Blackburn and Callard, ’97), achorioplacenta and choriovitelline placenta aretransitory features of placental ontogeny in Niveo-

scincus coventryi. A chorioallantoic placenta andomphaloplacenta become established in stage 34embryos and are maintained throughout the re-mainder of development. These two placental typespersist because the egg is physically divided intoembryonic and abembryonic hemispheres by acellular partition that connects the bilaminaromphalopleure to the yolk sac splanchnopleure(Fig. 13). The isolated yolk mass is a prominentfeature of the omphaloplacenta until stage 38 whenit becomes fragmented, presumably because of re-sorption. The morphology of the chorioallantoic pla-centa is uniform with the exception of a tendencyfor slight variation in the para-embryonic (equato-rial) region of the egg (Fig. 15). In specimens ex-hibiting this character, the differentiation occursonly on one side of the cross section of the egg,indicating that it is not distributed uniformly. Uter-ine vessels and the overlying squamous epitheliumprotrude slightly into the uterine lumen in this re-gion and cells of the chorionic epithelium are ir-regular shaped and larger than those over theremainder of the placenta. The apparent absenceof a shell membrane at all embryonic stages sug-gests that this structure is not prominent but lightmicroscopy may not provide sufficient resolutionto determine if a shell membrane is present. Al-though shell membranes of viviparous squamatesfrequently have been observed with light micros-copy (Blackburn, ’93a), confirmation of their ab-sence requires examination of ultrastructure.

Synopsis of placentation ofPseudemoia spenceri

Several of the definitive characteristics of pla-centation of Pseudemoia spenceri are shared withP. entrecasteauxii (Stewart and Thompson, ’94).These features include early loss of the isolatedyolk mass, early vascularization of the yolk sacsplanchnopleure, development of a cellular parti-tion that closes off the upper margin of the yolkcleft, and development of an allantoplacentalplacentome associated with the mesometrium.However, the morphology of the uterine epitheliumof the choriovitelline placenta and the regional dif-ferentiation at the equatorial margin of the chorio-allantoic placenta of P. spenceri are novel.

Our earliest embryos, stage 30, have a completeisolated yolk mass, a choriovitelline membraneand an extensive chorioallantoic membrane. Al-though the yolk sac splanchnopleure, bilaminaromphalopleure and isolated yolk mass are pre-sent, there are regional differences in each ofthese structures that reflect the ontogeny of the


Fig. 37. Pseudemoia spenceri. Chorioallantoic placentomeof a stage 38 embryo. ×430.

Fig. 38. Pseudemoia spenceri. Omphaloplacenta of a stage38 embryo. ×1165.

Fig. 39. Pseudemoia spenceri. Chorioallantoic placentomeof a stage 40 embryo. ×1105.

Fig. 40. Pseudemoia spenceri. Chorioallantoic placenta ina region peripheral to the placentome. Stage 40 embryo. ×1600.


Fig. 41. Pseudemoia spenceri. Embryonic stage 40. Zoneof contact between the chorioallantoic placenta and theomphaloplacenta. The arrowhead indicates the partition sepa-rating the two placental regions. The asterisk marks the lo-cation of the allantoic cavity. ×835.

Fig. 42. Pseudemoia spenceri. Omphaloplacenta of a stage40 embryo. The arrowheads indicate omphalomesenteric ves-sels in the yolk sac splanchnopleure. ×1240.

omphaloplacenta. The development of the ompha-loplacenta proceeds sequentially from the sinusterminalis in the para-embryonic plane of the eggtoward the mid-abembryonic pole. Thus, ontogeneticchange is retarded at the abembryonic pole com-pared to regions closer to the para-embryonic re-gion and, for stage 30 embryos, the morphology ofearlier stages of the development of the omphalo-placenta is apparent in the region of the abembry-onic pole. In this region, the isolated yolk mass isintact, the outer epithelial layer of the bilaminaromphalopleure is squamous, the uterine epitheliumis columnar but not ridged and yolk sac vessels areabsent. In contrast, splanchnopleuric vessels areconcentrated below the sinus terminalis and themorphology of the isolated yolk mass, bilaminaromphalopleure and uterine epithelium is modifiedin association with the splanchnopleuric vessels. Forregions of the omphaloplacenta that are vascular-ized by the yolk sac vessels, the isolated yolk massis absent, the epithelium of the bilaminar ompha-lopleure is cuboidal/columnar and the columnaruterine epithelium is ridged. This morphology is es-tablished for the entire omphaloplacenta by stage35 embryos and persists throughout gestation.

Of the three placental structures evident instage 30 embryos, the choriovitelline placenta istransitory, confirming a developmental patternthat is common for squamates (Stewart, ’93). Thechoriovitelline placenta has been considered to bea respiratory exchange system among squamatereptiles because of the close apposition of embry-onic and uterine vascular systems, which are typi-cally separated by squamous epithelial cells(Stewart, ’93). The cellular hypertrophy and fold-ing of the uterine epithelium (Figs. 23, 29, 30)has not been reported previously and suggeststhat the choriovitelline placenta of P. spenceri isa site of placental nutrient exchange.

The choriovitelline membrane is supplanted bythe chorioallantoic membrane prior to embryonicstage 35, yet the uterine epithelium adjacent tothe omphaloplacenta retains the morphology char-acteristic of the choriovitelline placenta. Theunusual characteristics of the choriovitelline pla-centa thus contribute to unusual regional differ-entiation of the chorioallantoic placenta, whichis still evident in stage 40 (term) embryos. Theuterine morphology also continues to extend tothe adjacent omphaloplacenta (Fig. 32).


Chorioallantoic placentation of P. spenceri is de-fined by three distinct zones differing primarilyin the morphology of the apposing epithelia. Thearea of uterine ridges adjacent to the omphalo-placenta is separated from the placentome strad-dling the mesometrium by a smooth region withapposing simple squamous epithelia. The cuboi-dal uterine epithelium of the placentome is foldedinto ridges overlying a network of blood vesselsand the apposing chorionic epithelium is alsocuboidal; features characteristic of type III pla-centation as defined by Weekes (’35).

One of the variable features of extraembryonicmembrane development of squamate reptiles isthe pattern of expansion of the allantoic vesiclein relation to the yolk sac (Stewart, ’93). For avariety of viviparous species (Stewart, ’85; Yaron,’85; Blackburn, ’93b; Stewart and Thompson, ’94,’96), the allantois is confined to the embryonichemisphere of the egg and the terminal yolk sacplacenta is an omphaloplacenta. In nearly all ofthese species, the allantois abuts a cellular par-tition that forms the upper wall of the yolk cleft.This partition, which is composed of intravitellinecells extending from the bilaminar omphalopleureto the yolk sac splanchnopleure, also occurs in N.coventryi and P. spenceri. Oviparous skinks lacksuch a partition and the allantois encircles theyolk sac in later developmental stages (Florian,’90; Stewart and Thompson, ’96).

A shell membrane, which is most obvious at theabembryonic pole of the egg, is present in stage 30embryos. This membrane is not apparent in stage35 embryos, although there is much material in theuterine lumen at the abembryonic pole, some ofwhich may be remnants of the shell membrane.

Identity of the placenta of“Lygosoma weekesae”

The placental description for L. weekesae wasbased on 32 specimens collected from the BlueMountains (approximately 200 kilometers west ofSydney, New South Wales) in December 1927(Weekes, ’29). Weekes originally believed thesespecimens to be Niveoscincus pretiosus (as Lygo-soma pretiosum), but after completing the placen-tal descriptions she realized they representedanother species (Browne, ’29; Kinghorn, ’29). Shealerted J.R. Kinghorn of The Australian Museum,who examined the four specimens assigned to L.pretiosum from the museum collection. Weekesalso returned to the site of the initial collectionof placental material and obtained six additionalspecimens. Kinghorn (’29) based his description

of Lygosoma weekesae on the four museum speci-mens, but appended a footnote to remark thatthe six specimens collected by Weekes did not dif-fer from the species description. These latterspecimens were deposited in The Australian Mu-seum. In a companion paper to Kinghorn’s (’29)species description, Weekes (’29) described pla-centation of L. weekesae. The specimens on whichthe placental descriptions were based have notbeen recovered (Rawlinson, ’74).

Four species of Scincidae that are likely sourcesof the specimens Weekes (’29) used to describe typeIII allantoplacentation in “Lygosoma weekesae” oc-cur in the region where she collected specimens:Pseudemoia entrecasteauxii, P. pagenstecheri, P.spenceri and Niveoscincus coventryi. P. entre-casteauxii and P. pagenstecheri are superficiallysimilar and were long considered to represent asingle species (Leiolopisma entrecasteauxii). �Leio-lopisma entrecasteauxii� has recently been rede-fined taxonomically as a complex of three species(Donnellan and Hutchinson, ’90; Hutchinson andDonnellan, ’92). Two of these species, Pseudemoiaentrecasteauxii and P. pagenstecheri, have type IIIallantoplacentae (Thompson and Stewart, ’94;Stewart and Thompson, ’96). The original descrip-tion of the allantoplacenta of “Lygosoma entre-casteauxi” (Harrison and Weekes, ’25) was basedon specimens from a locality outside the currentrange of P. entrecasteauxii. These specimens mostcertainly were P. pagenstecheri (Thompson andStewart, ’94). Additional specimens used in descrip-tions of placentation of “Lygosoma entrecasteauxi”were from localities where P. entrecasteauxii andP. pagenstecheri are sympatric currently (Weekes,’29, ’30, ’35). Only one of these specimens has beenrecovered, a female P. entrecasteauxii that was en-tered into the catalogue of The Australian Museumas “L. entrecasteauxi” (Thompson and Stewart, ’94).We think it likely that Weekes followed the con-vention of the time and considered all of these popu-lations to be “L. entrecasteauxi.”

Rawlinson (’74) examined all ten specimens re-ferred to by Kinghorn (’29) and placed L. weekesaein synonomy with the previously described spe-cies, Pseudemoia spenceri. Rawlinson’s (’74)interpretation that the material used by Weekes(’29) for the placental description could not havebeen P. spenceri was based on Weekes’ (’29) state-ment of clutch sizes. Weekes (’29) collectedfemales of both “L. entrecasteauxi” and “L.weekesae” on her collecting trip and noted, “... fe-males of each species carry from three to sevenyoung.…” (Weekes, ’29, p. 57). Rawlinson (’74) re-


ported an average litter size for a sample of 29 P.spenceri as 1.9 (range, 1–3). The range of littersizes for N. coventryi noted by Rawlinson (’75) was1–7 (mean = 3.0, n = 15). Because Weekes’ (’29)notes on litter size lie outside the range for P.spenceri, but not for N. coventryi, Rawlinson (’75)concluded that Weekes had described the placentaof N. coventryi (as L. coventryi). This interpreta-tion is not supported by our data. Our data forlitter size for P. spenceri do not differ from that ofRawlinson (’74) and based solely on Weekes’ (’29)statement of litter sizes, we concur that her speci-mens could not have been P. spenceri. Litter sizefor our sample of N. coventryi is somewhat lessthan for Rawlinson’s (’75) specimens, but does notrefute Rawlinson’s (’75) argument. However, theplacental description for “L. weekesae” cannot havebeen based on specimens of N. coventryi, becausethis species does not have a type III allanto-placenta. Because P. spenceri does have a type IIIallantoplacenta and Weekes made two collectionsof “L. weekesae” from the same locality (Kinghorn,’29), one of these collections identifiable as P.spenceri (Rawlinson, ’74), we believe it most likelythat Weekes’ (’29) specimens were P. spenceri andthat she erred in reporting litter sizes.

Comparative extraembryonic membraneand placental morphology among

Eugongylus group speciesThe ontogeny of extraembryonic membranes

and placentation have been described for five spe-cies of Australian Scincidae (Bassiana duperreyi,Niveoscincus coventryi, N. metallicus, Pseudemoiaentrecasteauxii and P. spenceri) (Stewart and Th-ompson, ’94, ’96) that have been assigned to theEugongylus species group (Greer, ’74, ’79). Bas-siana duperreyi is oviparous and the remainingspecies are all viviparous. Each of these speciesis uniquely identifiable based on the structure ofextraembryonic membranes and placentation, yetmany features are shared among some, or all, spe-cies. Variable characters include, reproductivemode, presence of a calcareous shell membrane,growth of the allantoic vesicle relative to the yolksac, structure of chorionic epithelial cells and al-lantoic vascular support of the chorioallantoicmembrane, structure of the epithelial cells of thebilaminar omphalopleure, and structure of theuterine epithelium in association with specific ex-traembryonic membranes.

A choriovitelline membrane, chorioallantoicmembrane and isolated yolk mass with a bi-laminar omphalopleure develop in all five species.

The choriovitelline membrane of all species con-sists of a rich array of omphalomesenteric ves-sels covered by a squamous epithelium. All specieshave at least one region of the chorioallantoicmembrane that is structurally similar to thechoriovitelline membrane, except that for thistissue a squamous chorionic epithelium covers al-lantoic vessels. Two characters distinguish Bassi-ana duperreyi from all other species, oviparity andexpansion of the allantoic vesicle such that thechorioallantoic membrane encircles the yolk sac.Expansion of the allantoic vesicle of the four vi-viparous species is restricted by a band of intra-vitelline cells that extends from the bilaminaromphalopleure to the yolk sac splanchnopleure.These cells form a partition, initially associatedwith the sinus terminalis, that divides the egginto two hemispheres. As a result, the yolk cleftand exocoelom form separate cavities throughoutdevelopment and the allantois does not extendbeyond the equator of the egg.

Among the four viviparous species, the uterinemorphology of N. coventryi is the least complexbecause a squamous epithelium occurs for all pla-cental regions. The only distinctive feature is theslight ridges that protrude into the uterine lu-men in the para-embryonic region because of thesuperficial placement of blood vessels. Theseridges are not uniformly distributed. The uterusof N. metallicus also contains ridges in the para-embryonic region, but these structures are uni-formly distributed about the equatorial zone ofthe egg and are larger and more numerous thanthose of N. coventryi. The para-embryonic regionof the chorioallantoic membranes of N. coventryiand N. metallicus contain enlarged, irregularshaped chorionic epithelial cells. These cells arequite distinctive in size and number in N. metal-licus and are distributed in a zone uniformlyabout the equatorial axis of the egg. The chori-onic epithelial cells of this region of N. coventryiare only slightly larger than epithelial cells fromother regions of the chorioallantoic membrane andthey occur as a narrow band that does not ex-tend around the circumference of the egg.

Pseudemoia entrecasteauxii and P. spenceri dif-fer from the other viviparous species in that theyhave a chorioallantoic placentome adjacent to themesometrium. The uterine component of theplacentome consists of vascularized ridges coveredby a cuboidal epithelium. The chorioallantoicmembrane is composed of a columnar chorionicepithelium with extensive vascular support fromthe allantois.


Although the choriovitelline membrane does notdiffer among species, the uterine contribution tothe choriovitelline placenta of P. spenceri is uni-que. The uterine epithelium of this species isfolded to form vascularized ridges.

The outer epithelial layer of the bilaminaromphalopleure is prominent in all four viviparousspecies. This epithelium is cuboidal in N. coventryiand cuboidal/columnar in the other species.Pseudemoia entrecasteauxii alone has extensivefolds in the bilaminar omphalopleure. The uter-ine epithelium of the omphaloplacenta of N.metallicus is columnar, which distinguishes thisspecies from N. coventryi. An omphaloplacentawith columnar uterine epithelial cells is also char-acteristic of both P. entrecasteauxii and P. spenceri,but unlike N. metallicus, the epithelium is raisedover vascularized ridges.

As is characteristic of oviparous Squamata, theegg of B. duperreyi is surrounded by a calcareousshell membrane. Differences in the presence of astructure identified as a shell membrane havebeen reported among the viviparous species(Stewart and Thompson, ’94, ’96). This structurehas been observed in N. metallicus and P. spen-ceri, but neither N. coventryi nor P. entrecas-teauxii. Because preparation of the material forontogenetic studies of these species was not in-tended to provide sufficient detail to analyze shellmembrane properties, these reported differencesshould be considered inconclusive.

“Eugongylus” allantoplacental typesWeekes’ (’35) analysis of allantoplacental cat-

egories included descriptions for five species cur-rently classified in the Eugongylus species group.Of the three types of allantoplacentae thatWeekes (’35) recognized (Table 1), the most spe-cialized types (II and III) occurred only amongthese five species. Of equal importance, none ofthe members of this species group had type Iplacentation as the sole morphology of the allan-toplacenta. Weekes (’35) constructed a morpho-logical series based on the three allantoplacentaltypes and, with an added inference of function,formulated a hypothesis for the evolution ofplacentotrophy. This hypothesis predicts that thethree placental types form a graded sequenceand makes additional predictions about the func-tional characteristics of each grade in the evolu-tionary sequence. Weekes’ (’35) scenario providedan influential model. Recent work suggests twoinherent weaknesses in the model: lack of a phy-logenetic perspective (Blackburn, ’93a) and de-

pendence on the allantoplacenta as the only siteof placental nutrient exchange (Stewart and Th-ompson, ’94, ’96).

Based on morphological criteria, specializationsfor nutritional provision to embryos among squa-mate reptiles are not restricted to a single site ofmaternal-fetal exchange, nor confined to a spe-cific time in ontogeny. Reconstruction of the evo-lution of placentation among squamate lineagesmust incorporate attributes of the entire interfacebetween female and embryo throughout gestation,and also must recognize uniquely derived featureswithin lineages. The constraints imposed by thestructure of the amniotic egg provided a suite ofplesiomorphic characters among species thatevolved intrauterine gestation (Stewart, ’97). Eachof the 100+ lineages of squamate reptiles thathave evolved viviparity initially shared these char-acters. In spite of the uniform functional at-tributes of the extraembryonic membranes anduterus, it seems unlikely that each evolutionarytrajectery has been identical. It does seem likelythat comparisons of placentation among vivipa-rous lineages will reveal a combination of plesio-morphic, apomorphic and homoplastic characters.The ontogeny of the extraembryonic membranesand placentation is known for only five species(one oviparous, four viviparous) of Eugongylusgroup skinks and a comprehensive phylogeny isnot available for this species group. An analysisof placental evolution that incorporates ontoge-netic trajectories is not yet feasible. However, be-cause of its preeminent influence as a model forsquamate placental evolution, we do think that itwill be instructive to discuss some of the implica-tions of our results for Weekes’ (’35) scenario inview of recent hypotheses for phylogenetic rela-tionships among these species.

Two species of Australian lygosomatine skinks,Pseudemoia entrecasteauxii and P. pagenstecheri(Hutchinson and Donnellan, ’92), in addition to P.spenceri, are known to have type III allanto-placentae (Weekes, ’35; Thompson and Stewart,’94; Stewart and Thompson, ’96). Greer’s (’89) tax-onomy included these species in two genera,Clareascincus entrecasteauxii (including P. entre-casteauxii and P. pagenstecheri) and Pseudemoiaspenceri. However, because both genera wereplaced in the same species subgroup, a single ori-gin of viviparity and placentation could be repre-sented. In a subsequent analysis of the genus“Leiolopisma,” Hutchinson et al. (’90) found closeaffinities among these three species (P. entre-casteauxii [as P. entrecasteauxii group 2], P.


pagenstecheri [as P. entrecasteauxii group 1] andP. spenceri) based on albumin MC’F and recog-nized three morphological synapomorphies for thegenus Pseudemoia. One of the synapomorphieswas “an advanced chorioallantoic placenta” (typeIII) as characterized by P. pagenstecheri (as P.entrecasteauxii group 1) (Harrison and Weekes,’25; Thompson and Stewart, ’94). Our results con-firm that P. spenceri shares this character.

The definition of type II allantoplacentation re-lied on the description of a single embryonic stageof Niveoscincus ocellatus (as Lygosoma [Liolepisma]ocellatum) (Weekes, ’30, ’35). Two additional spe-cies, N. metallicus and N. pretiosus, were assignedto this category, but placental descriptions were notprovided. The distinguishing characteristic of thisplacental category is the superficial placement ofuterine vessels, which bulge into the uterine lu-men to form folds (Weekes, ’30). The apposing chori-onic ectodermal cells are enlarged. Weekes (’30)described this morphology as occurring uniformlyover the surface of the chorioallantoic placenta.Stewart and Thompson (’94) noted that the chorio-allantoic placenta of N. metallicus conformed onlyin part to Weekes’ (’30) description because theallantoplacenta of this species is differentiated re-gionally. The site of uterine ridges and enlargedchorionic epithelial cells occurs only in a para-em-bryonic zone adjacent to the omphaloplacenta(Stewart and Thompson, ’94).

Weekes’ (’35) definition of type I allantopla-centation emphasized close apposition of mater-nal and fetal vascular systems resulting fromthin, flattened chorionic and uterine epithelialcells overlying allantoic and uterine blood vessels.This morphology occurred uniformly throughoutthe allantoplacenta. Weekes (’35) assigned sevenspecies of Australian Scincidae, none of which aremembers of the Eugongylus species group as cur-rently constructed (Greer, ’89; Hutchinson, ’93),to this placental category. With the possible ex-ception of two species (Weekes, ’30), the morphol-ogy defined as type I allantoplacentation occursin all viviparous Squamata, either as the solestructure or as one component of a regionally dif-ferentiated allantoplacenta (Blackburn, ’93a).Weekes (’35) considered type I to be the mostprimitive phylogenetically.

Two alternate taxonomic nomenclatures havebeen proposed for the three species representingtype II allantoplacentation, although each placesall three species in a single genus (Greer, ’89;Hutchinson et al., ’90). The species coventryi isplaced within Niveoscincus (species with type II

placentation) by Hutchinson et al. (’90), but in aseparate genus, which would require an indepen-dent origin of viviparity and placentation, by Greer(’89). Hutchinson et al. (’90) noted, in placing N.coventryi within the genus, that both immunologi-cal distance data and skeletal morphology sug-gested an independent lineage was represented.This interpretation was supported further by phy-logenetic analysis of allozyme data (Hutchinson andSchwaner, ’91). Among species of Niveoscincus, theontogeny of placentation is known only for N.metallicus (Stewart and Thompson, ’94) and N.coventryi. Neither N. coventryi nor N. metallicus(Stewart and Thompson, ’94) conforms to any ofWeekes’ (’35) allantoplacental categories. Most ofthe chorioallantoic placenta of both species doesmeet the specifications of type I placentation, butboth species also exhibit uterine and chorioallan-toic modifications over a para-embryonic zone ad-jacent to the omphaloplacenta. The morphology ofthe para-embryonic zone of N. metallicus, but notof N. coventryi, meets Weekes’ (’30) criteria for typeII morphology. The principal difference in allanto-placentation between these two species is in thedegree of modification of this region. Specifically,uterine ridges are more numerous and extend fur-ther into the uterine lumen and chorionic epithe-lial cells are larger in N. metallicus. In N. coventryi,these features are less well developed, appear in-consistently among specimens and are not circum-ferential in distribution.

The most recent phylogenetic analysis of theAustralian Eugongylus species group (Hutch-inson et al., ’90) places Pseudemoia (includingentrecasteauxii, pagenstecheri and spenceri) as asister group to a clade containing the oviparousgenera Bassiana and Morethia. The genus Niveo-scincus (including coventryi, metallicus, ocellatusand pretiosus) lies in a second clade with theoviparous genus Lampropholis as a sister taxon.Because the outgroup for the analysis, Emoialongicauda, is oviparous, and both of the aboveclades contain both oviparous and viviparousgenera, two independent origins of viviparity andplacentation are indicated. Thus, all of the spe-cies with type III allantoplacentae (Pseudemoia)are derived from an independent origin of vivi-parity from that of species with “Niveoscincustype” allantoplacentae. If this phylogenetic hy-pothesis accurately reflects the evolution of thesegenera, two uniquely evolved specialized allan-toplacentae occur among Australian Eugongylusgroup skinks and Weekes’ (’35) morphological se-ries is not an evolutionary sequence.


ACKNOWLEDGMENTSApproval for laboratory work was provided by

University of Sydney Animal Care and Ethics Com-mittee (approval nos. 90/LO4/20, LO4/193/3/646and LO4/10-95/2/2185). Animals were collected un-der permits B783 and B1137 from the New SouthWales National Parks and Wildlife Service. Speci-mens were deposited in the Australian Museum(N. coventryi R141345-347, R143477-481, R143451-454, R143491-492, R149709, R149741, R149744-745; P. spenceri R141277, R141279-286, R143483,R143493-495, R143457, R149708, R149710-711,R149716, R149732-737, R149739, R149746). Spe-cial thanks to Dan Blackburn, Simon Blomberg,Fiona Downey, Carol Esson, Dennis Greenwell, An-drew Krockenberger, Steve Morris, Rebecca Pyles,Kylie Russell and Lin Schwarzkopf.

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Documents

Placental ontogeny of the Australian scincid lizardsNiveoscincus coventryiandPseudemoia spenceri