Text-Book of Embryology 2-8 (1919)

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Kerr JG. Text-Book of Embryology II (1919) MacMillan and Co., London.

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Chapter VIII Modifications of the Envelopes and other Adaptive Modifications Occurring During the Early Development of the Amphibia

The Amphibians form a group of Vertebrates which have taken less or more completely to a. terrestrial existence in their adult condition. They have not been able to emancipate themselves entirely from the ancestral aquatic habitat, possibly on account of the feeble development of the horny outer layer of the epidermis. They are still as a rule entirely aquatic during the early stages of their development,the eggs being laid in water and the young animal passing its larval existence in the water.

In a number of cases, particularly in Anura inhabiting tropical regions with a well-marked dry season, very interesting adaptations are found whereby the young animal is enabled to pass a more or less prolonged period out of the water. In the first type of these adaptations we find special modifications of the tertiary envelope which is normally a simple mass of jelly deposited round the egg.

The first type of such adaptation is exemplified by various species of If;/locles and by Rana opistltorlon in which the eggs are simply deposited in free air in damp spots, each surrounded by a transparent spherical protective shell. In R. opisthoclon (Boulenger, 1890) the young Frog before hatching develops on the tip of its snout a small conical protuberance apparently used like the egg-tooth of Reptiles and the similar organ in Birds to tear open the egg-envelope. A further interesting adaptive feature. is that the young unhatched Frog possesses on each side of its body a series of vascular flaps of skin somewhat resembling the gill-flaps of an Elasmobranch fish and apparently functioning as respiratory organs.

In a considerable number of tropical Anura the ovidueal secretion which surrounds the eggs is, at the time of laying, beaten up by rapid movements of the hind feet of the parents into a fine foam or froth with numerous entangled air-bubbles. This may be deposited on the surface of a pool where it floats about like a fleck of ordinary foam with the developing eggs scattered through it (Paludiwla

fuscomacula/u.). At a particular stage in development a digestive

ferment apparently is secreted, probably by ectodermal gland cells, which liquefies the jelly and allows the larvae to drop through into the underlying water.‘ In other cases the mass of foam is deposited in an excavation in the ground, so situated that rain-water readily trickles into it (Engystrmza orale), or merely in a damp spot. In the case of the Japanese 1t’lz.ucoplo.orus (.Polg/pedates) so/Llegelé (Ikeda, 1897) the b11rrow is made in a bank by the margin of standing water and after the mass of egg-foam has been deposited the pair of Frogs make their way out by excavating a tunnel which slopes downwards and opens near the water’s surface. Here again at the appropriate stage. of development the jelly liquefies and the. young larvae are carried down by it into the water.

In the case of Phy/lloznedwsct 11.3/pot-/aonu’rial'£s the process of oviposition was observed by Budgett (1899) in the Gran Chaco. The eggs are deposited during the night, the female clambering up amongst the leaves of a suitable plant by the margin of a pool, with the male on her back (Fig. 208). With their hind legs the. two Frogs bend the margins of a leaf together so as to form a funnel into which the eggs are poured together with the fertilizing sperm. The eggs are enclosed in a mass of firm adhesive jelly which causes the leaf to retain its funnel shape. The eggs develop within the jelly up till

1 It is probable that. such ferments play an important part in softening the eggcnvclopes preparatory to hatching in various animals. Thus in Lcpidosircn the process of hatching is rendered possible by the softening of the egg-shell brought about apparently by digestive ferment secreted by the ectoderm covering the body (Graham Kerr, 1900). The same appears to he the case in Telcosts (Wintrebert, 1912). In Xrmbpus amongst Amphibians :1. similar process apparently takes place and in this case Bles (1905) attributes the formation of the ferment not simply to the diffuse activity of the cctodcrm cells but to the action of a special “frontal gland.” It seems not improbable that the formation of such hatching ferments will be found to occur very generally in aquatic Vertebrates.

Flu. ‘Z08. _I’/1,:/llunuv/ustt l1._c/pm:/u_;m.lr£u.l-is, l'e1u.ale carrying male on her back during oviposition. (After Builgttt, 1899.)

contain no egg in their interior? The eggs are thus protected both above and below by a thick mass of eggless spheres. During the later stages of development the layer of envelope next the surface of each egg becomes greatly distended by the accumulation of fluid within it, the jelly between the eggs meanwhile diminishing in volume. with their huge external gills have thus considerable room in which freely. Eventually the envelope ruptures and the larva hatches. comes to be occupied by a seething mass of tadpoles, floored and roofed in by a thick mass of jelly formed by the empty Eventually——in from 12-24

spheres.

the stage of a tadpole of 9-10 mm. in length. During this process the jelly apparently liquefies, until only a thin membranous bag containin g watery fluid surrounds each embryo. Eventually the remains of the jelly with its contained tadpoles trickles downwards into the water. of the water has retreated from immediately below the leaf the tadpoles may still make their way for a distance of several inches to the pool by active jumping movements, helped it may be by a shower of rain.

In the allied I’hg/llomedusa sauvagii, from the same neighbourhood, a similar mode of oviposition occurs, though here the nest is composed of several leaves (Fig. 209). Agar (1.909) finds in this case that both at the commencement and end of oviposition there are laid a large number of spheres of jelly which

'l‘he larvae to move

The nest thus

hours after the bulk of the larvae have

hatched——the jelly begins to deliquesee and the larvae drop down with it into

the water.

FM. 209. - -— I‘/lg/ll4ruu;du.w so I’ ('14;/ii, mass of spawn. (After Agar, l909J

Similar nesting habits occur in other tropical Hylids, e.g. Phyll0 1 In the eommon I‘-‘rocr j.’u-nu. /nu.-~m'-rm-in. a narent] em it ca wsnles ma be formed an I l.l y l. y l y

in quantity in the oviduct before eggs begin 10 enter it (Wezel, 1908). with the normal eggs Agar found 2-3 per cent nl'.s1u:l] eggless capsules.

Interspersed 'l‘hese appear

to be deposited round small solid particles such as fragments of shed epithelium

(Lebrun, 1891).

In C'671.t7'opho7'us, where the left ovary is no longer functional, empty tertiary envelopes are frequently still formed in the left oviduct (Braus, 1906).

If, as sometimes happens, the margin

I medusa vllterringvli (von Ihering, 1886), H3/la nebulosa (Goeldi, 1895),

Rltacopltorus 're7Jnwardt'£'13(Siedleeki, 1909). In the last mentioned the eggs are deposited in a mass of foam enclosed in one or several leaves (Fig. 210). At the appropriate time the central portion of the mass liquefies and the colourless tadpoles make their way into this central fluid —«—~ the superficial layer of the mass being hard and dry. Eventually the lower part of the mass softens and the

_1iq_uid containing the tadpoles trickles out on to the ground where

the larvae are able to continue their development in the smallest puddles. s

In the second type of such adaptations the eggs or young are carried about, away from the water, by one of the parents. In the simplest of such cases no structural modification of the parent's body is involved. Thus i_n A/,3/tes ribstetrwicans the male draws the strings of eggs out of the cloacal aperture of the female and loops them round his thighs-——the portion of oviducal secretion lying between successive eggs- becoming highly elastic and gripping the thighs tightly. Oviposition takes place on land and the male pays only occasional visits to the water. When one of these happens at the appropriate period the. young hatch in the form of tadpoles while the male parent resumes his terrestrial habits.

In a number of - cases the transport of the young by the parent takes place at a later period, when the tadpole stage has been reached, the larvae adhmhing to the back of the male parent and so being transported from one pool to another (Fig. 211, A). This habit occurs in various species of Iflzndrobates and P/b3/U0- p A _ bates (Brandes u. Schoenichen, 1901). 1“‘5,‘,;(,)_:§}g.' "fl£1’"(f)';?’t’f‘;f;El

In the most interesting cases however },,,M,,.,,’ ,p‘.“1‘I;‘f)1t,_\__ ‘(11,;,-W. 5,9,1. the transport of the eggs or young by the lvvki, 1909-) parent is associated with the making use of some particular structural feature of the latter——-either permanent or specially developed for this purpose- In Rhacophorus reticulatus (Uriiiitlxer, 1876) the eggs are carried about by the female, adherent to its ventral surface. In Iig/la goeldivl (Boulenger, 1895) the eggs adhere to the dorsal surface of the female, only in this case the skin of the parent responds to the stimulus afforded by the presence of the eggs and grows up into a slight ledge surrounding them (Fig. 211, B). In Pipe amcricana (Bartlett, 1896) the cloaca of the female is protruded at the time of oviposition as a large spout-like structure which projects forwards between the dorsal surface of the female and the ventral surface of the male. The eggs pass out one by one through this and are distributed at fairly equal intervals over the dorsal surface of the trunk of the female. The skin now proliferates actively, growing up so as to form highly vascular partitions between the eggs, each of the latter coming to be enclosed in a deep pit. The mouth of this becomes closed in by a dark-coloured operculum, possibly formed of hardened epidermal secretion. Each egg is thus enclosed in a little chamber in which it passes through the early stages of its development, including a modified tadpole stage, and issues forth eventually (after about 82ldays) as a young Toad.

In another set of Anurous Amphibians the eggs undergo their development in a spacious single cavity

 within the parental body. In Rhivzoderma

darwvlml (Jimenez de la Espada, 1872; Plate, 1897) this cavity is the enlarged unpaired croaking sac of the male, into

which the eggs, to .the number of from 5 to 15, are swallowed and from which the

young issue after completing the tadpole

, stage. In the genus Nototrema the brood

cavity is a special large pouch lying

beneath the skin of the back, lined by

involuted epidermis and opening to the exterior just in front of the cloacal aperture. In different species of the genus there is much difference in the length of time

during which the developing embryo is

retained within the pouch, the length of this period being apparently correlated with the size of the egg and the amount 1.~IG_ 21]__A, M1,, ,,,- [:/,.‘,/g[,_,_ of food-yolk stored within it. Thus in hates to-'initatz's carrying tad- N. vizarsupiatum there may be as many as P°‘;";',.?. "B! .f""“‘1f_°f ff’Jt"‘ 200 eggs in the pouch, each measuring ?;':',"',,'1',l.',",,_,.(,'.',i.1_‘ rjV§1;,%_i;"gb' ( er about 5 mm. in diameter (Brandes u. Schoenichen, 1901), and the young make their way out as typical tadpoles which doubtless lead for a time a free aquatic existence before metamorphosis takes place.

In N. omgfemm (Weinland, 1854) the eggs are much larger

(10 mm.) and fewer in number (about 15) and in this case as in the

. allied N. testudinemn and N. fls.s"ipes, which also possess large eggs,

the young go on developing within the pouch until after the period of metamorphosis.

In Noto/mama. an interesting adaptive feature characterizes the external gills. These organs are present upon branchial arches I and II, each consisting of a long slender stalk, which passes at its outer end into a thin highly vascular membrane formed by the fused and expanded outer ends of the two external gills. The two membranes so formed, one on each side of the body, are closely applied to the inner surface of the thin egg-envelope. The outer surface of the envelope is in turn in intimate contact with the highly vascular lining of the pouch which sends projecting folds in between the eggs. We have here clearly an adaptive arrangement to minister to the respiratory needs of the developing young, analogous with that provided by the allantois of a Reptile or Bird.

It appears somewhat puzzling that the eggs of Nototrema should come to be contained in a pouch the opening of which is much smaller than the cross-section of the egg. The probability appears to be (Boulenger, 1895) that the pouch is formed in response to the presence of the eggs upon the animal’s back, a ridge growing up round the eggs as in the case of fig/la. goelrlvli but in this case continuing its growth towards the mesial plane until the corresponding upgrowths from the two sides meet and completely roof in the pouchlike cavity. Support is given to this explanation by the condition in N. pg/gznacmn where the opening of the pouch is in the form of a median longitudinal slit, prolonged forwards as a kind of seamvor raphe along which the roof of the pouch readily tears and which presents all the appearance of having been formed by the coming together of two originally separate lips.

It must never be forgotten that such peculiarities of development as have been alluded to in the above--mentioned Anura involve adaptive modifications on the part of the young individual itself. The most frequent of such modifications is physiological adaptation, as shown for example by the fact that the transference of the young individual to water before the normal time is commonly fatal. In other cases structural adaptations of a more conspicuous kind are apparent. Thus in 1-’1Ipu. the late tadpole stage, although enclosed within its cell, develops a broad and highly vascular ta.il which doubtless serves for 1'espirato1'y and possibly nutritive interchange with the maternal tissues: again in ]Vutotrema, the external gills show the peculiar modification already alluded to. The true external gills are in several cases absent, their function being taken over by the vascular surface of the yolk, while in such a case as Rana opz'.s-thodcm. special new respiratory organs have been developed.

In the various modifications of development dealt with in the

‘preceding section we have to do with attempts, so to speak, on the

part of isolated members of a particular group of Vertebrates (Anura) to lessen the degree of their dependence upon the ancestral aquatic habitat. Such attempts amongst the existing Amphibia are not altogether successful: the group as a whole remains chained to a watery, or at least humid, environment.

The lower Vertebrates which made a real success of terrestrial existence, emancipating themselves entirely from the aquatic environment, are represented to-day by the Amniota, and it remains now to study their special modifications of development. 464 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

III. ADAPTIVE MODIFICATIONS IN THE DEVELOPMENT or THE AMNIOTA.-—It is characteristic of many Vertebrates that, associated with the provision of special arrangements for nourishing the young individual, the tiI11e of commencing an independent life on its own account is greatly delayed. In such cases where a considerable proportion of the whole development takes place within the shelter of the egg-shell (or of the parental body) we have to do with what is known as embryonic in eontradistinetion to larval development. During embryonic development the young individual is free from the necessity of fighting and lending for itself; it is to a great extent sheltered from the struggle for existence, and in correlation with this we find remarkable hypertrophies and modifications of various parts of the body taking place which in a free state would render life impossible.

The first of these modifications makes its appearance in the lower, aquatic, Vertebrates in the form of a pronounced bulging of the ventral side of the body. In the more primitive holoblastic Vertebrates this is caused by the great thickening of the ventral endoderm (Fig. 80, E, p. 146), its cells being much enlarged and packed with granules of yolk. Where this distension of the endoderm cells is most marked anteriorly there is brought about the tadpole shape of body as seen in the Ganoids and Lept'do.s7I7vm: or, on the other hand, the distended region may be situated towards the hinder end as in _l-’et7'0r/nyzon, Uemtodas or the Grymnophiona. In such cases as development proceeds the large yolk-cells go on segmenting, the yolk within them is gradually used up, and the Inass of endoderm, becoming Inore and more attenuated, ceases to project beyond the general outline of the body.

In the meroblastic egg, as has already been shown, the proportion of living protoplasm amongst the yolk has been reduced to vanishing point so that except superficially the yolk never segments. Typically it becomes gradually enclosed in the endoderm which spreads over its surface. There is thus formed what is known as the yolk-sac, a structure ‘usually of enormous size as compared with the rest of the embryo. lt will readily be understood how impossible a free active existence would be while there is a large yolk-sac present. The assimilation of yolk and its transport to the actively growing parts of the embryo are brought about mainly by the rich development of superficial blood-vessels forming the vitelline network. In typical Teleosts, ag. Salmonids, the yolk-sac becomes at an early period completely separated from the dorsal part of the endoderm which becomes the functional gut, the yolk absorption taking place entirely by the vitelline vessels.

An important point to be remembered is that the vitelline network though primarily nutritive in function is necessarily also respiratory, gaseous interchange taking place between the blood circulating in its vessels and the medium which bathes its surface. The vitelline network is the primary breathing organ in the great VIII DEVELOPMENTAL AD APTATION S 465

majority of Vertebrates during early stages of development. In cases where the embryo lies in contact with maternal tissues the respiratory exchange takes place ultimately, through the thin intervening layers of fluid or envelope, between the blood circulating in the vitelline network and that circulating in the oviducal lining of the mother. In this way all the necessary preliminary conditions are provided for the evolution of a placenta, and as will be shown later these conditions are actually taken advantage of in some cases and a simple yolk-sac placenta is formed.

In the more highly developed types of yolk-sac the splanchnic mesoderm which surrounds the vitelline vessels sprouts inwards, forming irregular vascular septa which project into the yolk-sac. This modification, which brings about a great increase in the assimilatory surface, reaches such a development in Birds that towards the end of incubation these ingrowths form an irregular meshwork of vascular trabeculae traversing the whole of the yolk right to its centre.

Eventually the yolk, whether in the form of a yolk-sac or a mass of heavily yolked cells, is enclosed within the ventral wall of the body. In the holoblastie Vertebrates this comes about as already indicated by the simple spreading of the blastoderm over the surface of the yolk so as completely to enclose it. In the Fowl the spreading of the blastode)rm, and its derivatives the endoderm and mesoderm, round the yolk is never quite completed, there remaining a small circular patch at which the yolk is separated from the albumen only by the remains of the vitelline membrane (of. Fig. 215, am).

Further in the Amniota the region of somatopleure bounding the coelomic space in which the yolk-sac lies becomes converted into amnion and serous membrane (of. Fig. 215, A), and is eventually cast off‘, playing no part iii the formation of the definitive body-wall. The yolk thus lies outside the limits of the definitive body-wall, projecting through the umbilical funnel which is bounded all round by the stalk of the amnion. Eventually, shortly before hatching, the edges of the umbilical opening are drawn over the yolk-sac in a manner which will be described later (see p. 475). In llacerta mlmlpztm in which the yolk-sac is reduced the remains of it are simply cast off according to Strahl.

The most remarkable of the excrescences adaptive to an embryonic existence are the organs known as Amnion and Allantois ——portions of the embryonic body which become greatly hypertrophied and perform important functions during embryonic life but which are eventually, for the most part, shed about the time of birth or hatching and play no part in the formation of the body of the adult.

AMNION.—-The most nearly primitive subdivision of the Amniota is the group Reptilia and we accordingly turn to it and more especially to the Ohelonia, which have been worked out by Mitsukuri (1891), to provide a foundation for our description.

VOL. II 2 II B

Fro. ‘.212.—-Chelonian blastodenns illustrating the development of the amnion. (A and C after Mitsukuri, 1891.)

A, ('Iemm.ys; 13, tfhelzudm; C, Ulemmys. a, amnion with neural rudiment seen indistinct.1_v through it; a.u., edge of amniotic flap; a.r, amniotic tunnel; c.g, cephalic groove; f, inconstant fold which is sometimes present; 31.1‘, gm-‘:trul8I' rim ; m._f, medullary fold ; gm, proamnion with head of embryo showing through it. *

466 CH. VIII DEVELOPMENT OF AMN ION 467

In Chelonia the iirst indication of amnion formation appears at a stage like that represented in Fig. 212, A. The future body of the embryo, indicated by the medullary folds, lies flat on the surface of the egg, extending out all round into the blastoderm. The first sign of the amnion is produced by thefront end of the medullary plate coming to dip downwards so as to form a deep slit or groove (Figs. 212 and 213, c._q) curving tailwards on each side as seen from above. '1‘he posterior wall of this slit forms the anterior limit of the head of the embryo while its anterior wall forms the rudiment of the amnion (Fig. 213, me). The portion of blastoderm in front of and to the side of the head of the embryo is as yet two-layered, the Inesoderm not yet having spread into it, and it follows that the amniotic ru(:1in1ont is also two-layered. This region of the blastoderm,

FIG. 213.——-Sagittal section through the head end of a Chelonian embryo. (After lVIit'sul<m'i, 1891.)

t.l..(:, anmiot;-iv ml;_-,-u : mgr, cephalic groove; earl, 4'-I.-t.m1r-1'-n1 of medullary plate ; end, oudoderm.

which is still without mesoderm and which in this case forms the amniotic rudiment, is termed the proamnion. As development proceeds the head end of the embryo increases in size and as it does so it dips more and more downwards so as to deepen the cephalic groove or slit in front of it. While this is going on there takes place active growth of the ectoderm along the sharp edge of the amniotic rudiment (Fig. 213, a.e) i.n such a way that this edge becomes prolonged backwards as a solid flap, covering over the body of the embryo from before backwards. This amniotic flap continues to grow tailwards, its growing edge concave and prolonged backwards on each side (Fig. 212, B, a.e), until it reaches the tail end of the embryo, so that the whole of the latter is covered in by an amniotic roof. Nor does the process stop now: it goes on with the result that there is formed a long tunnel (Fig. 212, C, a.t) continuous in front with the amniotic cavity, ~23.e. the cavity between 468 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

the body of tlie embryo and the amnion, a11d terminating behind in an opening bounded above by a concave free edge (a.e)._

An important point to realize is the relation of the amnion to the cell layers. The first rudiment, as has been indicated, is composed of the two primary layers ectoderm and endoderm, and this applies also to the lateral prolongations baclnvards of the free edge. The whole of the amniotic roof however except these marginal parts is formed at first of solid ectoderm and of ectoderm alone (Fig.

FIG. 214. —-—Diagrammatic transverse sections through Clielonian en1ln‘_\'ns (f-.'!v-m-nz.a/.s-. A, stage with 2-3 mesoderin segments; B, 6-7 segments) i1ln.s-1.r::t_i'n«,-; tlw rela1..ion.~e of tinamnion. (Based on figures by Mitsnkuri, 1891.)

a.f, amniotic flap; am, amnion: H", I'(:l.u¢lterm; rm/, vndoderrn; _/Zn, fnI.~u-_ amnion: 'nu-.<-, 1m-sods-.rm segment; N, notoehord; srI., so-ru-amniotzie junction; mm, somatoplenre: spl, sphmuhnupleme; splc, splanchnocoele.

214, A, a.f). As development goes on the mesoderm extends between ectoderm and endoderm and then splits into somatic and splanchnic layers. The result of this is that the endoderm, with its covering of splanchnie mesoderm, sinks down and no longer projects upwards on each side into the base of the amnion (Fig. 214, B). The somatic mesoderm on the other hand does continue to project into the base of the amnion just as did the endoderm previously (Fig. 214, B). The originally simple ectodermal roof of the amniotic cavity undergoes a process of splitting from its lateral margin inwards and as this split extends towards the mesial plane. the amniotic fold of VIII DEVELOPMENT OF AMNION 469

somatic mesoderm spreads with it. Except along the middle line the amniotic roof thus becomes double——-the inner roof being formed of ectoderm internally and somatic mesoderm externally, the outer roof of somatic mesoderm internally and ectoderm externally. Of these. two roofs the inner is the amnion (Fig. 214, B, am), the outer is the false amnion or serous membrane (f.a). The portion which retains its original condition of being formed of unsplit ectoderm (sa) may be. called the amniotic isthmus or the sero-amniotic connexion (Mitsukuri). During later stages of development this becomes reduced to a thin vertical partition in which form it persists throughout, except in the region of the head where it disappears entirely so that there is here a continuous coelomie space stretching from side to side between amnion and serous membrane.

’J‘he posterior tubular prolongation of the amniotic cavity becomes obliterated through part of its extent and in this way the amniotic cavity becomes completely closed.

The first-formed part of the amnion, lying in front of the head of the embryo, remains for a time proamniotie in character, '£.e. composed of ectoderm and endoderm, but eventually the mes'oderm and eoelomie. space spread in between the two primary layers and the portion ot' the amnion in question comes to resemble the rest.

As the body of the embryo becomes constricted oil’ from the yolksac the basal edge of the amnion continuous with that of the embryonic somatopleure becomes tucked inwards so that the amnion, which formed in earlier stages a mere roof, comes to form a complete envelope. The amniotic cavity is filled with secreted fluid in which the body of the embryo floats.

BIRDS.-——-The process of amnion formation i11 the Birds shows conspicuous dil‘f'erenees from that which has been described for the more primitive Reptiles. Two of the chief of these dili'ere.nces seem to be associated with the fact that the amnion develops relatively later in the Bird, at a period when the head and anterior body region of the embryo project prominently above the general level of the blastederm and when the mesoderm has already split into splanchnic and somatic layers. Correlated with this fact we find (1) that in the Bird the amniotic rudiment has to grow upwards so as to surround the projecting head and trunk, and (2) that the upgrowth is composed of somatopleure only.

The amnion may be said to originate as a kind of wall, formed of a11 upwardly projecting fold of somatopleure, which comes to surround the actual body of the developing embryo. This wall is not absolutely vertical: it is tilted, or inclined inwards, towards the middle of the embryonic body. With increasing growth it projects more and more over the body of the embryo, its free edge bounding a gradually diminishing opening, through which the body of the embryo is visible when looked down upon from above. Eventually this opening is reduced to vanishing point and the body of the embryo is completely 4'70 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

covered in by a double roof formed by the amnion and the serous membrane.

The amniotic fold does not develop with equal activity throughout its extent. Its growth is much more active anteriorly than elsewhere, with the result that the headward portion of the fold becomes extended rapidly backwards as an amniotic hood over the head and anterior end of the body of the embryo (cf. Figs. 233, 235, 236). '.l‘he last remnant of the amniotic opening is consequently situated quite near the hind end of the body.

Correlated with the later appearance of the amniotic hood——at a time when the coelomic cavities are extensively developed—-it is at no period composed throughout, from side to side, of a simple layer of unsplit ectoderm as was the case with the Chelonian. It is of interest to notice however that the sero-amniotic isthmus has not altogether disappeared, although it never has the breadth that it has in early stages in the Ohelonian.

The details of amnion formation are readily observable in the Fowl and have been fully described by Hirota (1894). The process takes place as follows: The iirst step consists in the appearance of a crescentic upgrowth of blastodcrm just in front of the head of the embryo at about the stage of 14 segments. At this period the mesoderm has spread forwards on each side but has not yet extended i11to the space immediately in front of the embryonic head (proamnion). Where the mesoderm is present it has split to form the coelome and owing to this being filled with secreted fluid the somatopleure bulges up somewhat so as to be conspicuously marked oil’ from the flat proamniotic area. The amniotic fold makes its appearance just about the anterior boundary of the. proanmion. As it increases in height it overlaps the head of the embryo and grows backwards over it as the amniotic hood (Fig. 233). Into the fold the mesoderm and coelomic cavities have already penetrated. Where the mesoderm from the two sides meet in the mesial plane of the hood the two portions of coelome do not open freely into one another but remain separated by a septum of mesoderm—-the mcsedermal sero-amniotic isthmus. At an early period of the backgrowth of the amniotic hood the ectoderm in the middle of its free posterior edge is seen to project headwards as a small wedge, the base of which is formed by the growing edge. As this wedge is carried backwards by the continued progress of the amniotic edge it leaves behind it a kind of trail in the form of a continuous line, or rather partition, of ectoderm connecting the ectoderm on the outer surface of the amniotic fold with that on its inner surface. This is clearly the ectodermal sero-amniotic isthmus of the Reptile persisting in a much attenuated form; the attenuation being due to the fact that the coelomic spaces have extended much nearer to the mesial plane than in the corresponding stage of amnion-formation in the Reptile.

Up till about the time when the amniotic hood has completed its backgrowth its cavity--—-the amniotic coelome—-—remains divided VIII DEVELOPMENT OF AMN ION 471

into two separate halves by a septum, which in front is purely mesodermal but throughout the rest of its extent is traversed by the ectodermal sero-amniotic isthmus. The anterior, purely mesodermal, part of the septum disappears early in the fourth day so as to make the amniotic coelome continuous from side to side, but the rest of the septum persists throughout the whole period of development although its central ectodermal portion becomes gradually reduced and by the tenth day has completely disappeared.

Towards the end of the second or early in the third day the tail of the embryo begins to project, bending ventrally and dipping downwards as it does so. As it does this the tail comes to be hidden under a projecting amniotic fold precisely as happened at the head end except that here the coelomic cavity is already completely continuous across the mesial_ plane there being no trace of a septum or sero-amniotic isthmus. The free edge of this “tail fold” of the amnion is, as was that of the “head fold,” concave only here the concavity is directed headwards. Early in the fourth day the concave edges of the head and tail folds become continued into one another at about the level of the hind limb rudiment, so that the body of the embryo is now surrounded by a continuous amniotic fold———most highly developed anteriorly where it forms the amniotic hood, less so in the caudal portion and least of all laterally. The more or less elliptical opening bounded by this fold, through which the dorsal surface of the embryo is exposed, gradually shrinks as the fold grows and eventually, during the first half of the fourth day as a rule, it becomes obliterated and the amniotic cavity closed.

The true amnion at first closely enshcaths the head and trunk of the embryo but from about the fifth day onwards watery amniotic fluid is secreted into its interior so as to form an extensive water jacket in which the embryo is suspended (Fig. 215). For a considerable period the embryo is gently rocked to and fro in the fluid by the slow rhythmic contractions of muscle fibres which develop in the somatic mesoderm covering the amnion on its outer surface.

The development of the amnion in the Sauropsida in general is adequately illustrated by the two types which have been described. There occur variations in detail. Thus the inequality in the activity of growth between the anterior and posterior portion_s of the amniotic fold so marked as a rule may be practically absent (Chameleons), or it may reach an extreme limit, the posterior

portion of the fold being obsolete and the anterior portion continuing

its backgrowth past the tail end of the embryonic body to form an amniotic tunnel, as in the Chelonians above described (Sphenodon, Gannet———S'ula, Puflin—Frate'rcula).‘

ALLAN'1‘oIs.———The allantois may also be conveniently studied in the Bird. In the Fowl it makes its first appearance as a little clear

‘ Schauinsland , 1906. 472 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

vesicle, projecting from the ventral side of the trunk near its hind

Fm. 215.-—~—Diug1-ams illustrating the arrangement of amnion, allantois, etc., in the Fowl. (After Lillie, 1908.)

A, fourth da '; B, ninth day. (1.0, amniotic cavity; alb, albumen; all, allantoic cavity; u.ll.st, allantoic stalk; am, amnion; coal, eoelome ; f.a, false amnion, or serous membrane; sa, seroamniotic isthmus; spl,splanchnop1eure; v.m, vit.elline membrane; 3/, yolk; 1, outer wall of allantois fused with serous membrane; 2, inner_wa1lofa1lantois. VIII ALLANTOIS 473

end (Figs. -239, 240), and serving for the reception of the renal secretion. 'l‘he study of sections shows that the allantois is simply a pocket of the ventral wall of the gut towards its hind end--correr sponding exactly with the bladder of an Amphibian. It is thus lined with endoderm and covered externally with splanchnic mesoderm. The allantois like the bladder of the Amphibian bulges into the splanchnocoele. As development goes on the allantois, distended with fluid, increases in size, projecting on the right or upper side of the embryo till it comes in contact with the inner surface of the soniatopleure (Fig. 215, A, all), and with still further growth flattens

out against the somatopleure taking a somewhat 1r_1_ushljopin-likeU

shape. In the case of an independently living airiiiiial such as an

- illlilc. 2l5A.~—-Diagram illustrating the arrangement of amnion, allantois, etc., in the Fowl. (After Lillie, 1908.)

C, twelfth day. ulb, albumen ; all, allantoie cavity; 3;, yolk.

Amphibian the allantoie outgrowth of the gut can only increase in size within the restricted space of the splanchnocoele which is already occupied by the viscera. In the Bird embryo on the other hand there are available for the growth of the allantois the wide—spreading extensions of the coelome, on the one hand between amnion and serous membrane and on the other over the surface of the yolk. The allantois accordingly spreads out all round towards the limits of this space (Fig. 215, B). As it does so it loses its rounded vesicular form, its proximal (Fig. 215, B, 2) and distal walls (Fig. 215, B, 1) approaching one another. The mesoderm covering its outer surface tends to undergo secondary fusion with that of neighbouring structures. Thus about the end of- the sixth day it fuses with the adjacent surface of the amnion. Again towards the time of hatching a similar fusion takes place with part of the yolk-sac. The most important of these fusions 474 EMBRYOLOGY OF THE LOWER VERTEBRATES 011.

however is that, which commences early in the fifth day, with the inner surface of the serous membrane. ' At a comparatively early period (during the fifth day) the mesoderm covering the allantois becomes vascular and as the organ Recoines flattened its proximal or inner and its distal or miter walls ecome strikingly different as regards their vascular-ity, the outer wall developing an extremely rich network of capillary blood_-ves§_<_:_s with very small meshes, while the inner wall possesses merely a gparse network together--.witl1 the large vessels of supply. This difference between the two walls of the allantois becomes conspicuous about the end of the sixth day of incubation in the common Fowl. The difference is associated with the fact that the distal wall of the allantois is destined to become the great respiratory organ, taking over this function from the vascular area of the yolk-sac by which it is performed during the early stages of development. In correlation with the more efficient performance of this function the albumen, or white, as it gradually shrinks in volume and acquires greater density gizavitates down to the lower side of the egg thus bringing the mushroom-shaped allantois close up to the shell membrane on the upper side. The process is still further facilitated by the ectoderm of the serous membrane becoming reduced to a very thin—-hardly distinguishable-—-layer in the region where it is underlain by, and fused with, the allantois. The capillary network thus comes into very close relation with the shell membrane and the overlying porous

' shell, and gaseous exchange can readily take place between the blood

circulating in the network and the external atmosphere.

As development goes on the respiratory needs of the embryo become greater and greater and these are met by the allantois spreading outwards all round its periphery, so as to provide a greater and greater respiratory area. During this spreading outwards of the allantois the three main alla11t(_;ic__ves_sel§ are somewhat retarded in their growth with the result that each one causes an indentation of the growing edge of the allantois beyond which the allantois bulges on each side.

When the growing edge of allantois comes, after about nine days’ incubation, into the neighbourhood of the remaining mass of albumen, a new phenomenon appears inasmuch as the allantoic margin with its covering of serous membrane proceeds to grow onwards close under the shell membrane as a circular fold recalling the amniotic

[old and enclosing the mass of albumen (Figs. 215, B, 215A, 0). l The"

ectodermal lining of the cavity so formed sprouts out into the albumen in the form of irregular projections which become vascularized from the allantoic mesoderm and no doubt play a part in absorbing the last remains of the albumen. By about the end of the second week of incubation the shell membrane is lined throughout the whole of its extent by the highly vascular outer wall of the allantois. This remains the breathing organ until-—a day or two before hatching—the y_gu_ng_chick_’s__l)eak vm DEVELOPMENTAL ADAi>rxrioy_s 475

/‘=r":“T _ , _ _ penetrates the air_-space and'\ ulmonary breathmg\ begins. The

-«up-w

all_ant9_i_c circiilation then gradua ffiecoines sluggish and stops, and eventually by a process of autotomy the allantois is separated from the body of the embryo and is left behind as the vascular membrane seen lining the fragments of ‘shell from which a young bird has hatched. '

ENCLOSURE OF YOLK—SAC WITHIN THE EMBRYONIO BODY.--—As already indicated thF3fi)1K—:sac~heco'mes eventually (about a couple of days before hatching in the case of the common Fowl) ,e_nclosed within the body-wall. The process by which this is brought about appears to be as follows (H. Virchow). With the growth of the embryo a great increase takes place in the area over which the amnion is fused with the proximal wall of the allantois (cf. Fig. 215A, C), the compound and highly muscular membrane so formed extending eventually almost completely round the yolk-sac. At its edge it is continued onwards by the somatopleure, this latter terminating round the circular area where the yolk remains exposed. The yolk-sac is thus contained in a space the wall of which is formed of the following components in sequence starting from the body of the embryo : (1) amnion, (2) amnion fused with proximal wall of allantois, (3) proximal wall of allantois and (4) somatopleure in the region of the distal pole of the yolk-sac. The proximal portion of this wall, being formed of amnion, is necessarily continuous with the body-wall of the embryo at the umbilical opening and further those parts of it formed from amnion and allantois are highly muscular and contractile. l)uring the later stages of development this wall slowly contracts and as it does so the yo1k—sac is pushed into the umbilical opening which closes after it.

_._EV QRIGIN.- QE.S|l11E.A.Ml>IION .——-As regards this question,

which has excited much controversy, the following appears to the

present writer to be the working hypothesis which fits most easily the facts so far as they are known.

(1) The amnion originated as a fold of l)l€t.'s't0d6’7'?Il« round the body of the embryo (Fig. 216, A, B).

As has already been shown the amnion arises in this way in ontogeny in the Reptilia which are generally recognized as being the most primitive Amniotes. The same holds for the Birds and for some of the Mammals.

The Mammalia as a group are admittedly descended from ancestors in which the egg was large and meroblastic as it is in the Reptilia. This is indicated, apart from other convincing evidence, by the fact that they still exhibit in ontogeny a well-d_eve_loped though yolkless

_Jt follows then that it iswiiiadniissilile to regard facts derived from the study of certain mammals in which the mode of amnion formation during ontogeny is of a‘ different, even though apparently simpler, type as constituting important evidence in regard to the phylogenetic origin of the amnion, as has been done in particular by Hubrecht (1895). vm DEVELOPMENTAL ADAPTATIONS 475

’‘v’?.‘’‘__'_‘ .5 .-.-.4-..»—.~-..~. u-...—oc-no-‘Q

_ _»_M ,_ A _~_ V__ . penetrates the air-s ace and(g11lgngn,ary ,b(r_e,a_t_hi(ng) her ms. The

allantoic c1rculatioii”t en gradua y ec6i:11ss"lsmggisii*a;’iiH”st'bps, and

eventually._by“a process of autotomy the allantois is separated from the body of the embryo and is left behind as the vascular‘ membrane seen lining the fragments of ‘shell from which a young bird has hatched. ‘

ENCLOSURE or YOLK-SAC WITHIN THE EMBRYONIC BoDY.—~—As already indi.cated t1i?yo1k?sae"b“eeo‘nies eventually (about a couple of days before hatching in the case of the common Fowl) _e,ncl_gserl within__thye_”_(_bo‘dy;wal.l. The process by which this is brought about appears to be as follows (H. Virchow). With the growth of the embryo a great increase takes place in the area over which the amnion is fused with the proximal wall of the allantois (cf. Fig. 215A, 0), the compound and highly muscular membrane so formed extending eventually almost completely round the yolk-sac. At its edge it is continued onwards by the somatopleure, this latter terminating round the circular area where the yolk remains exposed. The yolk-sac is thus contained in a space the wall of which is formed of the following components in sequence starting from the body of the embryo : (1) amnion, (2) amnion fused with proximal wall of allantois, (3) proximal wall of allantois and (4) somatopleure in the region of the distal pole of the yolk-sac. The proximal portion of this wall, being formed of amnion, is necessarily continuous with the body-wall of the embryo at the umbilical opening and further those parts of it formed from amnion and allantois are highly muscular and contractile. During the later stages of development this wall slowly contracts and as it does so the yolk-sac is pushed into the umbilical opening which closes after it.

.,_E mQm:mJmAMmQ§mAs regards this question, which has excited much controversy, the following appears to the present writer to be the working hypothesis which fits most easily the facts so far as they are known. .

('1) The amnion originated as a fold (pf blastoderm round the body of the embryo (Fig. 216, A, B).

As has already been shown the amnion arises in this way in gnatggeynypiri the Beptilia which are generally recognized as being the most primitive Amniotes. The same holds for the Birds and for some of the Mammals.

The Mammalia as a group are admittedly descended from ancestors in which the egg was large and meroblastic as it is ‘in the Reptilia. This is indicated, apart from other convincing evidence, by the fact that they still exhibit in ontogeny a well-dey_e_lgpe,,d‘_,thgugh yolkless

t follows then that it is inadmissible to regard facts derived from the study of certain mammals in which the mode of amnion formation during ontogeny is of a‘ different, even though apparently simpler, type as constituting important evidence in regard to the phylogenetic origin of the amnion, as has been done in particular by (Hubrecht (1895). on. V11‘ ‘EV/OLUTION” OF AMNLION 477

(2) The amniotic fold consisted at first of yolk-sac wall, the body of the embryo being forced down into the yolk-sac as it increased in size, possibly by the resistance of the rigid protective shell associated with the assumption of a terrestrial habit.

The tendency towards predominant development of the anterior portion of the amniotic fold may probably be correlated with the predominant growth and ventral llexure of the head end of the body which would cause it to dip down into the yolk-sac particularly markedly.

The delay in the appearance of mesoderm in the region of the proamnion may similarly have been originally due to the pressure of the downwardly flexed head,

(3) The yolk-sac with its richly developed superficial network of blood-vessels was the respiratory organ of the embryo at this early phase in the evolution of the Amniota. It follows that the portion of it nearest the shell, and therefore in the most favourable position for carrying on the breathing function, would tend to increasein area and would therefore bulge more and more over the body of the embryo (amniotic fold, Fig. 216, A, B, gt. f) so as eventually to utilize the whole of the inner surface of the shell.

(4) The egg being new terrestrial the excretory poisons produced by the activity of the already functional renal organs could no longer pass away by diffusion into the surrounding water. It would obviously be disastrous were they to accumulate in the space round the embryo and they therefore had to be retained within the body. This led to the great and precocious enlargement of the receptacle for these excretory poisons, already present in the pre-anmiote ancestor, the allantoic bladder.

(5) This precocious enlargement of the allantois in turn necessitated the early increase in size of the coelomic cavity to accommodate it.

(6) The allantoic wall-—-a part of the gut-wa1l——was naturally vascular, like the rest of the gut-wall, and with its great increase in size it would come in contact with the inner surface of the somatopleure. But as soon as it did this respiratory exchange would take place between its b1ood—-—through the substance of the somatopleure ——and the medium outside. The allantois would thus constitute a new, though at first small, breathing organ.

(7) As the embryo grew its respiratory needs would grow also. Meanwhile of its two respiratory organs the one—~-the yolk-sac-— would be shrinking in size and therefore diminishing in efficiency while the other—the allantois———would be increasing in size as it became more and more distended. This would lead to the supplanting of the yolk-sac by the allantois as the main respiratory organ. As the allantois increased in size it would tend to extend in the position of greatest respiratory efliciency, 13.6. close under the somatopleure.

(8) With the development of the allantois and coelome the splanchnopleure would be freed from the somatopleure and the 4'78 llll\lliI'i-1r().I40(iiY OF THE LOWER VERTEBRATES OH.

upgrowth round the body of the embryo-—the amniotic fold- —would now become purely somatopleural (Fig. 216, C).

(9) As soon as the amniotic fold extended so far over the body of the embryo as to roof it in completely it would at once assume a new importance in protecting the delicate body of the embryo, enclosed wi.thin it as in a Water jacket, from the dangerous jars and shocks incidental to a terrestrial existence. In correlation with the importance of this function of the closed amnion we might expect to find a tendency for its closure to be accelerated. As a matter of fact i.t will be found that in various mammals, including man, the amniotic cavity is closed from the beginning. ,

——-In many different groups of animals the embryonic phase of development is passed within the oviduct (uterus) of the mother. The advantages of this are obvious, for not only is the young individual sheltered to a great extent from the struggle for existence, as it is even Within an egg-shell, but it forms for the time being as it were part of the body of a complete adult individual with its full equipment for holding its own in the struggle. It is in the group Mammalia amongst Vertebrates that viviparity reaches its highest development, as the final touch in their adaptation to a terrestrial existence, but it is of interest to notice that the phenomenon occurs, in a less highly elaborated form, here and ,.~,(,_ 21;_.---,.,-.H..._,__,.m.,“ Ur there amongst the lower Vertebrates-——Fishes, A(?(I../If/H'II..s‘Hi5llClOSl1lg‘ Amphibians, and Reptiles. t‘V"°«‘=’%"- (The ‘mi’ Thus amoncr the Elasmobranch fishcsl there sious of the scale rc- _ ° . .

I_n.emt millimetres.) are numerous genera in which the early stages 1Il development are passed through in the uterus.

In such cases we find in the first place a well-marked tendency towards the reduction of the protective egg-envelopes which are no longer necessary. Thus there is found, as a rule, during early stages a typical set of egg-envelopes, but the horny shell is very thin and weak as compared with that of oviparous Elasmobranchs and as development goes on (embryo of 7-8 cm. in Acantluias) it becomes still

thinner, breaks up, and disappears.

A curious feature in such cases is the tendency for a group of eggs to be enclosed in a common set of envelopes instead of each egg having its own set. Thus in Acanthvlas (Fig. 217) there are commonly from two to six eggs enclosed in a common shell; in Trygonomvine two or three, in It/uinoliatus seven or eight.

The embryo within the uterus is still nourished primarily by the yolk in its yo1k_-sac. This primitive mode of nourishment has not

1 See Gudger, 1912.

IV. Vrvrmnrrr IN THE LOWER VERTEBRATES.Vin VIVIPARITY IN FISHES 479

yet been replaced by a process of Etbsorpiiloll from the uterine wall as is the ease in the Mammalia. But the uterine wall already plays a part though a minor one in providing food material for the young individual by its glandular activity. The beginnings of this are seen in the albuminous fluid enclosed within the egg-shell, and it is possible that the elongated gill-filaments of the embryo play a part in absorbing nourishment from this. A further development consists in the secretion of an abundant “uterine milk ” which is drawn into the pharynx through the spiracles by precocionsly occurring movements like those of respiration and passed on into the digestive tract.

In accordance With its glandular activity the lining of the uterus frequently undergoes an increase of area by growing out into villi or trophonemata (Wood -Mason and Alcock, 1891). In the StingRays specially enlarged trophonemata may be drawn into the pharyngeal cavity of the embryo through its greatly dilated spiracles so that their secretion reaches the alimentary canal of the young fish directly (Fig. 218).

During the later stages of intrauterine development there usually comes about an intimate relationship between the surface of the yolk-sac and that of the uterine lining and in associ.ation with this there is found 3' Varying degree of Fro. 218. -—-Portion of uterus of Pterospecialization Of the uterine llI1- plataca. 7aiz:ru:ra slit open to show an ing (Ercolani, 1879; Widakowich, f"":?"3'r°.‘l":t')“. 1907). This latter may be smooth ,:,,l,p1(Ai1';,.,;|’{, 1g._;9i:)

(Squatina angelus, Notitlomus cine mas), or project into longitudinal folds so as to give increase of surface (Acantlvias oulgamls, .«S'c3/mnus lz'c}m'a), or grow out into papillae or trophonemata (Torjoeolo, Pteroplataea). finally it may develop folds which interlock with grooves on the surface of the yolk-sac, the uterine and yolk-sac surfaces being iii the most intimate contact so as to constitute physiologically a definite yolksac placenta (0'cm'chcm3as glcmcus, Mustelus laemls, etc.). _ p

Amongst the Teleostean fishes viviparity occurs occasionally, in at least half-a-dozen different families; the Cyprinodontidae, Seorpaenidae and Embiotocidae furnishing the greatest number of cases. They are particularly numerous amongst the Embiotocidae and Scorpaenidae of the western coast of North America (Eigenmann, 1894). i 480 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

The eggs are retained in the ovary, either in the follicle, or in the cavity of the ovary; more rarely in the dilated oviduct or uterus. The developing embryo may depend for its nourishment upon the yolk (Scorpaenidae); it may absorb nourislnnent by the surface of the yolk—sae which grows out into villi (Anableps); or the nutritive secretion of the ovarian wall may be taken into the alimentary canal and there digested (Embiotocidae).

Among the Amphibia true viviparity is rare. A well-marked case occurs in Salmnanrlm atm (Wiedersheim, 1890). Here a large number (40-430) of eggs pass into the oviduct when breeding is about to take place but of these all except the one (in rare cases as many as four) next the eloaeal opening Simply break down forming a kind of broth which fills the oviducal cavity. The embryo nourishes itself, after it has used up its own yolk supply, by gulping down and digesting this fluid, which contains not merely the yolky debris of disintegrated eggs, but also large, quantities of red blood corpuseles derived from extensive haemorrhages of the uterine wall.

Perhaps the most striking feature of the Mammalia is the ex't1‘e1ne degree of adaptation which they typically show to an intra-uterine mode of development in which the embryo leads a parasitic existence attached to the uterine lining of the mother. In accordance with this the external ectoderm of the blastocyst becomes modified to form organs of attachment which eventually, in the region of the yolk-sac and more especially in the region of the allantois, become vascularized and elaborated i11to the complex nutritive and respiratory organs named placentae. This being so, it becomes of much interest to enquire whether amongst those Amniota which are lowest in the scale of evolution——the Reptiles—— there are any foreshadowings of the type of adaptation to intra-ute.rine development found in the Mammalia. Probably numerous such cases exist but at the present time, with our extremely imperfect knowledge of Reptilian development, we are acquainted with only a-few. The most interesting of these is that of the Italian Lizard Ohalcides ttrlidactylus (Seps clmlcides). Giacominfs description of this (1891) may be said to form the foundation of what will one day probably form an important chapter in Vertebrate embryology.

The eggs, which measure about 3 mm. in diameter, are first found in the oviducts early in May, while the first young are born towards the end of July, the period of gestation thus being between one and two months.1 The eggs become spaced out along the oviduct or uterus, so as to give it a moniliform appearance, each egg being arranged with its apical pole towards the mesometrium. At about the Iniddle'of gestation the “egg” presents the appearance shown in Fig. 219, A, the whole forming a kind of blastocyst about 7 mm. in diameter. The outer surface is formed by the ectoderm of the serous membrane. Within the serous membrane there can be seen the allantois with transparent, richly vascular, wall and the yolk 1 About sixty-five days according to Mingazzini. vm EMBRYONIC ADAPTATIONS IN REPTILES 481

szw, more opaque than the allantois and already much smaller than the latter as seen in surface view. The edges of the allantois and the mushroom-shaped yolk-sac fit closely together and between them is the body of the embryo contained in the amnion. As in other Sauropsidans or Prototherian Mammals the yolk-sac lies on the embryo’s left, the allantois upon its right—-upon the side, in this case, next the mesometrium. As development proceeds the exposed area of yolk-sac becomes gradually reduced by the encroachment of the allantois. The latter however remains merely in contact with the edge of the yolk-sac and never comes to "surround it. Over the yolk-sac area there remain visible, for a long time the remains of the vitelline membrane (cf. Bird). Both allantoic and yolk-sac regions of the surface develop placental arrangements, the former being physiologically the more important of the two:

The allantoic placenta is already becoming apparent at the stage shown in Fig. 219, A, in the form of an elliptical area at the mesometrial pole which adheres to the uterine lining by means of numerous little projections which interlock with similar projections on a corresponding uterine area. As de- L up velopment goes on the egg assumes an elongated shape (Fig 219, The whole of FIG. 219.--“ Egg": of ()'[urlcides trzdactylus. the uterine lining in contact (After (‘”“’°"i“"" 1891')

with the outer surface of the A, 7 mm. in diaineter, showing yolk-sac (3/.3), allan “ n - - - . _ - tois (all), and foetal portion of allantoic placenta (pl); ego ls Provlded “nth 3' nch B, 15-16 mm. in longest.-diameter, seen from apical

O

capillary network C1033 pole,slm\\'in;.:I'«n-1:11portionufnllauitoic pla.centa(pI). beneath the uterine epithe lium and here and there insinuating itself between the epithelial cells. Over the allantoic placental area the maternal projections now form undulating ribbons attached along one edge and free along the other. On the surface of these ribbons the uterine epithelium instead of being flattened as it is elsewhere is columnar and has a glandular appearance. With the r.il)ho11-like projections just mentioned there interlock the somewhat similar projections of the foetus. These are also covered with columnar epithelium close under which lies a rich capillary network. The latter is not confined 482 EMBRYOLOGY OF THE LOWER VERTEBRATES OH.

to the actual placental projections for even the smooth parts of the surface over the allantoic area are provided with an extraordinarily rich network of capillaries which show an even more marked tendency than those of the uterus to penetrate into the epithelium. Over the smooth area the foetal and maternal surfaces are in intimate contact, so that the two capillary networks lie parallel and close to one another, separated only by two very thin epithelial membranes. In the region where foetal and maternal projections interlock chinks are apparent between the two in which there appear to be traces of a fluid 1naterial—-—probably nutritive and secreted by the maternal epithelium which as already mentioned has in this region a glandular appearance.

The. yolk-sac placenta is less highly developed. In the region of the centre of the yolk-sac flattened ridge-like projections also appear which interlock with corresponding uterine projections and become vascularized as the mesoderm spreads beneath them. Between the two surfaces is the remnant of vitelline membrane but this gradually disappears so that foetal and maternal surfaces come into intimate contact.

(7/utlcides ((}(m.gg/lus) ocellatus, another Italian lizard, is also viviparous and in it occur similar though less marked adaptations to viviparity (Griacomini, 1906). Here in the later stages of gestation the general arrangement of the foetal envelopes resembles that in 0’. t9~'o'dwctg/lus. The allantoic region of the foetal surface is smooth and possesses a rich capillary network. It lies in immediate contact with the uterine lining, which in this region is covered with very thin flattened epithelium overlying an extremely rich network of maternal capillaries.

.The portion of uterine lining in relation with the vitelline region of the foetal surface is less richly vascular, is covered with thicker epithelium of vacuolated cells with large nuclei, and is thrown into low folds which interlock with corresponding folds of the foetal surface so as to form an incipient yolk-sac placenta. The foetal epithelium of this region is thickened and in places columnar and appears to have an absorbent function. As in 0. tmldactg/lus remains of membrane are to be seen for a time between the foetal and maternal surfaces in this region.

To sum up, we find in Chalcicles ocellatus a less advanced stage of adaptation to intra-uterine development than in C’. tmidactylus. Probably similar conditions will be found in various other viviparous Lizards as e._q. in the Australian Tmcltysaurus and T'il'iz1ua scincoides (0'ycloclus botidccerti) (Haacke, 1885).

In the Blind-worm (Angwis fm_q'il'is) and in the Viper (Vipem) and Smooth snake (C'oronella austrriaqa) viviparity also occurs but here in a still more definitely incipient form—a thin shell persisting throughout development and the foetal envelopes and uterine lining remaining practically unmodified.

Thus in the three sets of Reptiles above mentioned we see three steps in the evolution of viviparity: VIII EMBRYONIC ADAPTATIONS IN REPTILES 483

(1) the mere retention of the egg within the uterus, the shell still remaining and no intimate relations being developed between foetal and maternal tissues (Angwis, Vipera, Uoronella),

(2) the rupture at an early stage and eventual disappearance of the shell, and the coming into intimate relations of foetal and maternal tissues, both becoming highly vascular and there being an attempt at the formation of a yolk-sac placenta (C'halc7Ides ocellatus), and (3) the development of an allantoic placenta (0. tridactylus).

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