Book - Vertebrate Embryology (1913) 8

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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.

Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures
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Chapter Viii The Foetal Membranes Of The Mammalia

The same foetal membranes are found in the Mammalia as in the Reptiles and Birds, namelj', the chorion or false amnion, the true amnion, the yolk-sac, and the allantois. Here, however, the yolk-sac, except in the Monotremata, is devoid of yolk ; but it has the same anatomical relations, as an appendage of the gut, as in the other groups, and in its splanchnopleure are developed the blood-vessels of an area vasculosa supplied by a pair of vitelline arteries from the aorta, and by a pair of vitelline veins which enter the hind end of the heart. The size of the yolk-sac varies greatly in the different orders of Placental Mammals. The allantois is always found, though its cavity may be very greatly reduced. Its function is, in the Placental Mammals, to carry the foetal blood-vessels to and from the placenta. It may also act as a receptacle for the waste products of the foetus. Its stalk inside the body of the embryo always persists as the bladder. The amnion is chiefly of interest owing to the peculiarities of its mode of formation in PlacentaUa. The false amnion or chorion, enveloping as it does the embryo and these other foetal membranes, naturally comes into contact with the uterine wall, except in the Monotremata, and so brings about that relation between foetal and material tissues which constitutes a placenta. Its outer ectodermal layer is known as the trophoblast.

Monotremata (Fig. 122, a)

The formation of the foetal membranes has not so far been described, but from the persistence, over the back of the embryo, of the connexion between the false and the true amnion, it may be gathered that the amnion was formed, as in the Sauropsida, by folds of ectoderm and somatopleure. Yolk-sac and aUantois are both large, the former lies on the left, the latter on the right of the embryo.

Fig. 122. - Foetal membranes of A, Monotremata ; b, c, d, Marsupials. B, Phalangista, Aepyprymnus, Didelphys, Bettongia ; c, Dasyurxis ; D, Perameles and Halmaturus. (In Didelphys the proamnion persists as in Dasyurus.) (a, b, d, after Semon; 0, after Hill.)

In this and the following diagrams of Mammalian foetal membranes the trophoblast (ectoderm of false amnion) is stippled, the ectoderm of the true amnion represented by a continuous line, the endoderm by a broken line, and the mesoderm (somatopleure and splanchnopleure) by a thick line swollen at intervals. aZZ., aUantois ; am.c, amniotic cavity; pr. proamnion ; y.s., yolk-sac ; s.t., sinus terminalis of area vasculosa.

Marsupialia (Fig. 122 b, o, d)

We do not know the mode of origin of the amnion, but it is to be presumed, from the description of the structure of the blastocyst in Dasyurus, that the amnion is formed from folds, and that these arise at the edge of the embryonic area. The inner layer of the folds - true amnion - would then be derived from the embryonic area, while the outer layer - false amnion - would come from the trophoblastic area, of which the other part of the blastocyst is composed.

There is generally a large proamnion - that region of the amnion below the head of the embryo from which mesoderm is absent - and this may persist {Dasyurus, Didelphys, Perameles).

In all Marsupials the yolk-sac is very large, its upper wall invaginated by the embryo. Between the lower (distal) wall and the trophoblast the mesoderm never completely extends, is absent in fact in the anti-embryonic half of the blastocyst, and in the mesoderm the extra-embryonic coelom never extends further than the line where the proximal turns over into the distal wall of the yolk-sac. In the mesoderm of the yolk-sac there is an area vasculosa, and at the extreme edge an annular vessel - ^the sinus terminalis. The allantois is always small. In some cases - Didelphys, Aepyprymnus, and others - it altogether fails to reach the false amnion, and its blood-supply is very poor. In others, however {Phascolarctos, Halmaturus, Perameles), the allantois is larger, reaches the trophoblast, and possesses well-developed blood-vessels, which in Perameles vascularize a placenta. This placenta, as we shall see below, has a very peculiar stru'cture.


The first of the foetal membranes to claim our attention must be the amnion, for though in a Placental Mammal the amnion may ultimately be formed by folds resembling those of the Bu-ds and Reptiles, yet this is not always so ; and even when that method does obtain, there is very good reason for supposing it to be not primary, but secondary.

In the Reptiles, Birds, and Monotremes we have seen that sooner or later, with the final enclosure of the yolk by the blastoderm, the false amnion comes to be a completely closed sac, in which lie the embryo in its amnion, with its yolk-sac and allantois. The cavity of the sac is extra-embryonic coelom ; its wall consists of an outer ectodermal and an inner somatopleuric layer.

In Placental Mammals this condition is realized almost in the first moment of development, for the first act of differentiation is the separation of the inner mass from the outer layer, and, as we are now to see, the inner mass contains within itself the material for the embryo, its amnion, yolk-sac and aUantois, and, we may add, the somatopleure and splanchnopleure of the extraembryonic coelom, while the outer layer or trophoblast is the representative solely of the ectodermal covering of the false amnion. It cannot be too often insisted that all Placental Mammals pass through this stage in which the material for the embryo with its membranes is shut up inside the sac of the trophoblast.

The next step is the separation in the inner mass of the embryonic knob- which comprises the material for embryo and amnion- from the lower layer- from which alimentary canal, yolk-sac, and aUantois are derived. The lower layer quickly grows round the inside of the blastocyst.

There follows the formation of the amnion ; two main types of which may be distinguished. In the first of these the future amniotic cavity is never open to the exterior, and the trophoblast over the embryonic knob persists. In the second the cells of the trophoblast overlying the embryonic knob- the so-called cells of Rauber- disappear, the embryonic knob comes up to the surface and the amnion is eventually formed by folds in a manner resembling that seen in Reptiles and Birds.

Of each of these types there are again two divisions.

I. (a) In the first division of the first type the future amniotic cavity never opens into any other cavity at all ; it arises either inside the embryonic knob (Cavia), or between that and the trophoblast {Erinacells).

(b) In the second division are those cases in which the amniotic cavity, developed in the embryonic knob, is in transitory communication with another cavity formed in the thickened overlying trophoblast. The connexion is soon lost and the trophoblastic cavity disappears {Mus, Arvicola).

II. (a) In the first division of the second type the embryonic knob is gradually folded out and becomes the embryonic plate. This is seen in Talpa, Tupaia, Vespertilio, Sus, Tarsius.

(6) In the other division the embryonic knob simply flattens out without the formation of any depression, and so becomes the embryonic plate {Learns, Ovis, Sorex).

As a good example of type I (a), we may take the guinea-pig iCavia) (Fig. 123). After the separation of the lower layer the embryonic knob begins to move away from its original pole of attachment to the opposite end of the oval blastocyst, as it does so pushing the lower layer in front of it. This lower layer represents here the upper wall of the yolk-sac only (and the alimentary canal of the embryo), the lower wall being never formed in the guinea-pig. The margins of the lower layer remain attached to the trophoblast at the original embryonic end. When the embryonic knob has reached the opposite end a cavity appears in it ; this will be the amniotic cavity. The cells lining this cavity are at first columnar, but a difference soon appears between those next the yolk-sac and those on the side facing the original embryonic pole. The former remain columnar and represent the embryonic area of the upper layer of a Reptilian blastoderm ; from these cells the blastopore, archenteron, notochord, and mesoderm presently originate, the remainder being the ectoderm. The latter soon become flattened and represent the inner part of the extra-embryonic blastoderm of the Reptiles ; this wall of the cavity becomes the (true) amnion. Meanwhile the germinal layers have been formed and the mesoderm soon extends outside the embryonic region into the space between the amnion below, the trophoblast above, and the yolk-sac at the sides ; a cavity appears in this mesoderm which is the extra-embryonic coelom, and is continuous with the coelomic cavity in the embryo. Where this mesoderm covers the trophoblast it is somatopleufe, where it hes over the upper wall of the yolk-sac it is splanchnopleure, and where it passes over the amnion it is somatopleure again.

The development of the embryo continues, and its body is folded oflE from the amnion, the fine of attachment of amnion to body-wall being brought continually nearer the mid-ventral line of the embryo and the umbiUcus so narrowed. The gut of the embryo has meanwhile been folded ofi from the yolk-sac, the stalk of which passes through the umbihcal aperture.

The trophoblast has now essentially the same relations to the yolk-sac, embryo, and amnion, as has the false amnion of tho chick when the blastoderm has completely invested the yolk. In the chick at this moment the false amnion is a completely closed sac enveloping the embryo in its amnion with its yolk sac ; in the guinea-pig the trophoblast does the same, rhe differences are due to : (1) the absence of a lower wall to the yolk-sac ; (2) the restriction of mesoderm and coelom to the re-ion between the trophoblast, the upper wall of the yolk-sac, and the embryo in its amnion ; and (3) to the invagination of the embryo into the upper wall of the yolk-sac.

Fig 123.- Formation of the amnion in the guinea-pig (Caw'a). (After Selenka.) a, early, B, later, c, latest stage, allantoidean (placental) trophoblast; o.Jrf.omphaloidean trophoblast ; lacuna ; e.fc., embryonic knob ; am.c, amniotic cavity ; y.s., yolk-sac.

Thus the amniotic cavity has developed as a cavity closed from its very inception.

The method of amnion formation in Monkeys and Man is not yet known, but it seems very possible that it is according to this first type.

The absence of the lower wall of the yolk-sac led, many years since, to a curious misinterpretation of the development of the guinea-pig, which w^as known as the ' Inversion of the Germinal Layers '. Early investigators missed altogether the trophoblast in the region of the yolk-sac- which is indeed thin and closely adherent to the uterine tissues - and then found the emhryo in its amnion enveloped in a membrane which was - as we now know - the upper wall of the yolk-sac. This membrane they traced into the alimentary canal and so called it endoderm. At the other end of the blastocyst the same membrane was found adherent to the trophoblast - here thickened in connexion with the development of the placenta - and believed to be continuous with it. The wall of the blastocyst was therefore endodermal and the germinal layers had in some mysterious fashion become turned inside out. The mistake was cleared up by the researches of Selenka.

Fig. 124. - Formation of the amnion in thehedgehog (Erinacevs.) (After Hubrecht.) ir., trophoblast ; y.s., yolk-sac ; e.k., embryonic knob ; cwi., amnion ; I., lacuna ; ec, ectoderm of embryonic plate ; n., notochord ; m., mesoderm ; c, extra-embryonic coelom. a, early, b, later stage,

Although in the hedgehog the amniotic cavity is not formed in quite the same way as in the guinea-pig, yet it never opens to the exterior or into any other cavity. The embryonic knob becomes detached in its centre from the trophoblast, while remaining adherent to it by its edge. The space between the two will be the cavity of the amnion. As the space enlarges the embryonic knob becomes transformed into a ciirved plate of columnar cells - the embryonic plate - ^the edges of which are rather thinned out. The cavity continues to enlarge and the thin edges, of flattened cells, grow up and in between the trophoblast and the cavity, so forming a roof to the latter. This roof is the amnion. In the meantime the coelom has been formed and extends up with the amnion between the trophoblast and the embryo (Fig. 124).

I. (b) In the mouse, rat and field-mouse the same invagination of the embryo into the upper wall of the yolk-sac that we have seen in the guinea-pig also occurs, and is indeed found, though not necessarily at this early stage, in all Rodents. Here, however, the distal wall of the yolk-sac is complete. Further, the embryonic knob never leaves the trophoblast at the original embryonic pole, but is driven to the other end of the blastocyst by a great thickening of the trophoblast, which is associated with the formation of the placenta (Fig. 125).

Soon a cavity appears in the embryonic knob - the amniotic cavity - and this immediately comes into communication with a cavity in the trophoblast. As soon, however, as the extraembryonic coelom is formed it extends into this region, forces the trophoblast away, and severs the connexion. The trophoblastic cavity disappears. That developed in the embryonic knob then becomes the amniotic cavity in precisely the same way as in the guinea-pig.

It is evident that here also the amnion is derived from the material of the embryonic knob, that the trophoblast is the homologue of the ectoderm of the false amnion alone.

II. {a) (Fig. 126). A depression appears in the embryonic knob. By the disintegration of the overlying trophoblast cells this depression comes to open to the exterior. The embryonic knob increases in size, the depression becomes wider and shallower, and the knob - or, as we may now call it, the embryonic plate - finally comes up on a level with the surface of the blastocyst. It is inserted by its edges into the surrounding trophoblast.

The amnion is formed by folds, which arise at the boundary of the embryonic plate. It is difficult to be certain, but it seems that the outer layer of the fold, that is, the false amnion, arises from the trophoblast, while the inner or true amnion comes from the embryonic plate itself.

II, (6) The amnion is formed in precisely the same way in this division ; there is, however, no folding out of the embryonic knob : it becomes directly flattened to form the plate. The overlying cells of Rauber disappear (Fig. 127).

Fig. 126. - Formation of the amnion in Tupaia (an Insectivore). a-e, Five stages ; e.p., embryonic plate ; R., cells of Rauber. Other letters as before. (After Hubrecht.)

Fig. 127. - Formation of the amnion in the rabbit (Lepus) ; i.m., inner mass ; l.L, lower layer ; e.p., embryonic plate ; B., cells of Rauber. (After Assheton.)

In this second type - where the amnion is eventually formed by folds, the tail-fold often arises, as in Tarsius and the rabbit, before the head-fold (Fig. 128). Otherwise the resemblance between the way in which this membrane of the foetus is developed iu Placental Majunials and that seen in other forms is certainly very close, and this similarity has suggested that this is the primitive method of amnion formation in Placental Mammals - as distinct from the rest of the class - and that those cases (type I) where the amniotic cavity appears inside the embryonic knob are secondary modifications due to the restricted space in which the blastocyst developes. It is perfectly true that the blastocyst is compressed by the narrow limits of the space in which it is fixed in Erinacells, Cavia, and Mtis. To the proposed hypothesis there is, however, one fatal objection, and that is the presence of Rauber's cells in members of the second type, the existence of a stage in which the material for the embryo and amnion is wholly enveloped in a closed sac, which is the homologue of the false amnion of Reptiles, Birds, and Monotremes. This is a stage through which all Placental Mammals pass, and in an Insectivore, some Rodents, and possibly in Monkeys and Man, the sac of the trophoblast remains closed. In others, after the disappearance of Rauber's cells, the embryomc knob comes to the surface and amniotic folds are developed. We are justified, therefore, in regarding the first condition as the primitive, the second as derived from it, and in supposing that one of the first effects of that loss of yolk which we know the ovum of the Placental Mammals has undergone, as compared first with the Marsupials and next with the Monotremes, was a precocious separation of the material for the embryo and its amnion from a closed sac, inside which the differentiation of the germ-layers and of the other foetal membranes could take place, secure from the pressure exerted by the contraction of the uterine walls. The loss of the shell, still retained by the Marsupials, may have been a contributory cause. Such a development has been preserved in those cases where the uterine cavity, or that part of it wherein the embryo is lodged, is narrow ; where it is wider reversion to the original method of forming amniotic folds has taken place, vmless we prefer to regard this as an independent piece of evolution. (The possibiHty of such an independent evolution is shown of course by the existence of amniotic folds in other animals, for example, in Insects.) Since the depression which marks the beginning of the folding out of the embryonic knob is probably the remains of the closed amniotic cavity seen in type I, the complete series illustrates the process of reversion to the original MammaUan method, while the distribution of type II over several orders (Insectivora, Cheiroptera, Ungulates, Rodents, Primates) is sufficient evidence of its repeated and independent occurrence.

Fig. 128. - Further stages in the formation of the amnion in the- rabbit. (After Van Bcneden.), head amnion fold ;, tail anmion fold ; e., embryo; aZZ., allantois ; a.<r., allantoidean trophoblast ; y.s., yolk-sac ; olir., omphaloidean trophoblast ; c, extra-embryonic coelom ; s.t., sinus terminalis of area vasculosa.

The Yolk-sac and Allantois

In the extent to which they are developed these vary greatly in the different orders.

In Rodents (Figs. 123, 125, 128) the yolk-sac is always large, and its upper wall sooner {Cavia, Mus) or later {Lepus, Sciunis) invagmated by the embryo. The mesoderm never extends fiu-ther than the edge of the upper wall. In the sj)lanchnopleure there is a well-developed area vasculosa, the blood-vessels of which convey to the embryo the nutrient materials absorbed by the yolk-sac from the uterus in the following way. The distal (lower) wall of the yolk-sac always disappears (in Cavia it is never formed) and then the lumen of the yolk-sac communicates freely with the uterine cavity. In the fluid contained in this cavity is tho protoicl and fatty material secreted by tlie uterine glands ; tliis is absorbed by the highly columnar, folded epithelium of the upper wall of the yolk-sac. The allantois is never large but may be quite well developed. In the mouse and guinea-pig, however, the ejiidodermal outgrowth of the hind-gut is confined within the limits of the embryo's body, and only the vascular splanchnople\ire passes out through the umbilicus

Fig. 128*.- Area vasculosa of the yolk-sac of the rabbit. (After Van Beneden and Julin.) ViteUine veins black, vitelKne artery and smus terminalis stippled.

and across the extra-embryonic coelom to convey the foetal blood-vessels to and from the placenta.

In the Camivora (Fig. 129) the yolk-sac is smaU compared to the aUantois, and of no importance in the later stages of development, though large and no doubt functional at first. So also in the Ungulates (Fig. 130). Of a fair size at first and provided with an area vasculosa, it is rapidly outgrown by the allantois, which attains enormous dimensions (Fig. 131), extending from one end of the elongated chorionic (trophoblastic) sac to the other, and occupying a very considerable space in the uterus. In its cavity are found floating large oval bodies, often very hard, known as hippomanes. These, as well as the fluid contents of the allantois, we shall describe when we deal with the physiology of the placenta.

Fig. 129. - ^Foetal membranes and placenta of the dog. m., mesometrium ; pi., zonaiy placenta. Other letters as before. (After Duval.) Foetal blood-vessels in black in this and the following diagrams.

Fig. 130. .- Foetal membranes of the horse, early stage. (After Bonnet.) Letters as before.

Fig. 131.- Foetal membranes of the horse, later stage. (After Bonnet.) villus; eiJ.i^., epithelial thickenings of amnion ; A., hippomanes. Other letters as before.

In the pig and sheep the tapering ends of the allantois and of the chorion which covers them, the so-called ' diverticula allantoidis ', undergo a partial degeneration. They are sharply marked off from the main body by an annular thickening provided with a sphincter muscle.

In the Insectivora (Fig. 132), again, the yolk-sac is usually large in early stages of gestation, and its blood-vessels may actually (Tupaia) begin to penetrate the placental thickening of the trophoblast. But as the allantois dcvelopes it drives away the yolk-sac, and the importance of the latter is diminished, though it remains till the time of birth. In Talpa, Erinacells, and So7-ex, the lower (anti-embryonic) wall of the yolk-sac is never covered by mesoderm. In Sorex the yolk-sac contains a bright green pigment, which is either biliverdin or nearly related to it, and is derived from the digestion of extravasated maternal corpuscles which have been eaten by the phagocytic trophoblast.

Fig. 132. - Foetal membranes and placenta of the hedgehog. (After Hubrecht.) l.u., lumen uteri ; d.r., decidua reflexa. Other letters as before.

In the Cheiroptera (Fig. 133) similar conditions are found. The yolk-sac, large and important in early stages, is later replaced by the allantois.

Of the Edentate foetal membranes we know little, except that in the sloth Choloepus the allantois is small, the amniotic cavity large enough to obliterate the extra-embryonic coelom, while the yolk-sac in the stage described has vanished. The same obliteration of the extra-embryonic coelom seems to occur in the Cetacea {Orca) and Proboscidea (ElepMs).

Fig. 133. - Foetal membranes and placenta of the bat ( Vespertilio). (After Nolf.) Letters as before.

In the Sirenia (Halicore) the aUantois is as large as in Ungulata, extending to both ends of the chorionic sac.

In respect of their foetal membranes the Primates faU sharply into two groups.

In the Lemuroidea (Fig. 134) the yolk-sac seems to disappear early, while the aUantois is very large, as in an Ungulate ; the placenta also, as we shall see, is of the Ungulate type. In the Authropoids-Moiikeys and Man-with which we must associate Tarsius- nsnany classified as an aberrant Lemur- the arrangement of the foetal membranes is utterly unlike anythmg found, anywhere else amongst the Mammalia. The yolk-sac is dimmutive but vascular (Fig. 138), the endodermal cavity of the allantois is smaU and almost confined within the body of the embryo, its splanchnopleure alone passing out to the placenta, while the extra-embryonic coelom is precociously developed.

Fig. 134.- Foetal membranes of the lemur. (After Turner.) v., villi. Other letters as before.

In Tarsius the complete history of these membranes has been given to us by the researches of Hubrecht (Fig. 135). As a result ' of a proliferation of cells at the hinder end of the embryonic plate a sac is formed lying posteriorly between the trophoblast and the small yolk-sac. This sac is the extra-embryonic coelom. Very quickly it extends until it occupies a very large proportion of the blastocyst. Where it covers the lower wall of the yolk-sac it is, of course, splanchnopleure ; elsewhere, applied to the trophoblast, somatopleure. There is at present no mesoderm between the embryonic plate and the yolk-sac. Thus this extra-embryonic mesodermal sac is well formed before the middle layer exists in the embryo. The yolk-sac remains small. Meanwhile, the amniotic folds have appeared, the tail-fold first, and a.s this grows forwards over the back of the embryo a solid cord of mesoderm is left connecting the hind end of the embryo with the trophoblast. The amnion closes, the embryo is developed and folded over inside it, and the cord - which carries the allantoic or umbilical blood-vessels and is indeed the umbilical cord - moves round until it is inserted into the original anti-embryonic pole of the trophoblast. It is here that the placenta is formed. The base of the cord is penetrated by the rudimentary allantoic outgrowth of the hind-gut.

Fig 135.- Development of the foetal membranes in Tarsius. (After Hubrecht ) a., blastocyst before Rauber's cells have disappeared ; b, the embryonic knob (e.k.) is being folded out to the surface ; the yolk-sac is complete ; c, the embryonic plate (e.p.) is at the surface, the extra-embryonic coelom (c) is formed ; d, the tail-fold of the ammon is growing forward, the allantois (all.) has penetrated the mesoderm of the bodystalk, a placental thickening has been developed at the anti-embryonic pole • e, the amnion is closed and the body-stalk or umbilical cord [u.c.) is shifting its position, to be attached to the placenta (pi.).

Fig. 136. - Two stages in the development of the foetal membranes in a monkey [Cercocebus). (After Selenka). Letters as before.

Fig. 137. - Early human cmbrj'o with its membranes. (After Peters.) d.h., dccidua basalis (serotina) ; d.r.ep., uterine cjiitholium covering the dccidua refloxa or capsulaiis ; I., lacuna in trophoblast (tr.) ; (il., \itorine gland ; m.b.v., maternal blood-vessels opening here and tliore into lacunae ; cl., clot niai-king (probably) the point of entrance of the blastocyst ; licro the cpitlicliuiu is interrupted. Other letters as before.

Fig. 138.- a, Longitudinal section of older human embryo. The allantois has grown out and penetrated the base of the body-stalk (6.s<.) (future umbilical cord). At the hind end of the embryo is a large blastopore (socalled neurenteric canal) leading into the yolk-sac. The gut has hardly yet been folded off. Underneath the medullary plate and in front of the blastopore is seen the notochord. (This figure should be compared with that of the bat, Fig. 93, a.) h.v., blood-vessels in the splanchnopleure of the yolk-sac. Other letters as before.

B. Transverse section of the body-stalk in the plane indicated at b in a. Above is seen the amniotic cavity, below the aUantois, and at the sides the umbilical arteries and veins. (After Graf Spee.)

The mutual relations of the membranes in Monkej's and Man are similar, but their origin has not yet l^een seen. We do know, however, that in the earliest human embryo yet described (Fig. 137), and in the corresponding stage observed in Monkeys (Fig. 136), there is a large extra-embryonic coelom, a smaU yolk-sac attached firmly to the lower side of the embryonic plate which itself forms the floor of a small cavity, the future amniotic cavity, the roof of which is the amnion. The embryo in its amnion with its attached yolk-sac is suspended to the somatopleure of the trophoblast by a short cord or stalk of mesodermal tissue. In this the umbilical artery and vem ^^n\l be developed, while the rudimentary allantois will penetrate its base (Fig. 138). This cord is the so-called ' ventral stalk ' ; but it is not in this early stage ventral in position, but rather posterior and dorsal. Body-stalk (Minot) would be a preferable term. Though we do not know the precise mode of developmsnt of these structures, it would probably not be too hazardous to surmise that the amniotic cavity has been formed, as in the guinea-pig, inside the embryonic knob and not by folds, that the extra-embryonic coelom was developed with the first formation of mesoderm, and that the body-stalk is the attachment left between embryo and trophoblast when this cavity spreads under the yolk-sac and over the amnion. The allantois - or rather the umbilical cord - would not then grow out to reach the trophoblast, for it would have been united with it ab initio.

Fig. 139. - Human foetal membranes and placenta. (After Balfour, after Longet.) The amniotic cavity {am.c.) has enlarged and occupies nearly the whole of the extra-embryonic coelom (c), being reflected over the umbilical cord (u.c.) and yolk-sac {y.s.). d.b., decidua basalis (serotina) ; d.r., decidua capsularis (reflexa) ; d.v., decidua vera ; l.u., lumen uteri ; am., amnion ; pi., placenta ; o.d., oviduct.

Later on the umbilical cord shifts its insertion on to the ventral side of the body of the embryo, as the hinder end of the latter is folded off inside the amniotic cavity. It retains its original point of union with the trophoblast, for the placenta is formed on this side.

In later stages of gestation the amniotic cavity is greatly enlarged, and the extra-embryonic coelom suppressed. The remains of the yolk-sac is thus squeezed up against the umbilical cord, and the whole invested by the amniotic epithelium (Fig. 139).

{The literature will be found at the end of the following chapter.)

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Jenkinson JW. Vertebrate Embryology. (1913) Oxford University Press, London.

Vertebrate Embryology 1913: 1 Introduction | 2 Growth | 3 The Germ-Cells, their Origin and Structure | 4 The Germ- Cells, their Maturation and Fertilization | 5 Segmentation | 6 The Germinal Layers | 7 The Early Stages in the Development of the Embryo | 8 The Foetal Membranes of the Mammalia | 9 The Placenta | Figures

Cite this page: Hill, M.A. (2021, May 5) Embryology Book - Vertebrate Embryology (1913) 8. Retrieved from

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