Book - An Introduction to the Study of Embryology 4

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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.
Haddon 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

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This historic 1887 embryology textbook by Haddon was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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Chapter IV. General Formation of the Body and Development of the Embryonic Appendages

The three germinal layers, the development of which lias now been traced, constitute the rough material, so to speak, for the further building up of the embryo. No new formative tissue will make its appearance, and it now remains to follow the further history of these layers. Before this can be done in detail, it is necessary to gain some idea concerning the formation of the embryo as a whole, and of some of the various secondary structures which are often associated with larval or foetal life.

In all those forms possessing a small amount of food-yolk, such as the Ccelenterata, Echinodermata, most Vermes, a few Mollusca, Amphioxus, Lamprey, and Amphibia, the embryo lias been carried to a stage which may roughly be stated to consist of an oval or rounded body with usually a single layer of epiblast. The primitive stomach or archenteron is lined with a single layer of hypoblast, and opens to the exterior by the usually posteriorly situated blastopore. The archenteron is more or less surrounded by mesoblast, which, as has just been shown, may have a single or a multiple origin.

Ccelenterata - Radial Symmetry. - The Hydroids may, in general terms, be said never to advance beyond this stage. In the fixed forms, which may be regarded as tentaculate gastrulse, the mesoblast is merely represented by the inconspicuous structureless lamina, the gelatinous tissue of the medusoid forms with its stellate cells, clearly having relation to their mode of life. The development of many Hydroids is obscured by abbreviation.

The Actinozoa can also be briefly dismissed, but they arrive at a higher stage of evolution than the Hydrozoa. A further ingrowth of epiblast takes place at the blastopore, so that a mouth and oesophagus lined by epiblast are formed. Such an epiblastic ingrowth is known as a stomodseum. The walls of the body are further symmetrically and bilaterally infolded. The cavity of the body (archenteron) is thus divided into a number of diverticula or pouches, separated by mesenteries, which primarily extend to the wall of the depending oesophagus. The epiblast (ectoderm) does not enter into the mesenteries.

The Actinozoa have advanced beyond the purely gastrula stage by acquiring a stomodseum and persistent archenteric diverticula. The compound and skeleton-producing forms exhibit no real advance upon this plan. Hseckel maintains that the Scyphomedusse and Actinozoa are offshoots from a primitive branch (Scvphopolypi) of the Coelenterata.

Fig. 68. - Ideal Section through the Long Axis of a Sea-Anemone.

The sides of the mouth and oesophagus are supposed to be appressed together, leaving only the two extremities open, which, in this case, form two channels of communication with the temporary stomach.

b.c. inter-mesenterial chambers or body-cavity: m. edge of mesentery; ces. oesophagus or stomodteum ; st. temporary stomach, formed by the contiguous upper digestive edges of the mesenteries ; t, t. axial tentacles in longitudinal section. The mesoderm is merely represented by the line between the ectoderm and endoderm.

It is a very significant fact that, so far as is known at present, digestion takes place in the Actinozoa only by means of the enlarged edges of the mesenteries. When food is introduced into the body, the edges of the mesenteries close round it and thus form a temporary stomach, which, for the time being, is cut off from the archenteric diverticula. This “stomach- communicates with the exterior by the elongated mouth. The latter is often temporarily constricted at the sides, merely leaving an orifice at each end, which simulates a mouth and anus, as shown in fig. 68. Wilson and others, appreciating these facts, have speculated upon the possible origin of the higher Metazoa from such a primitive form.

Formation of Body-Cavity. - In the Echinodermata a distinct advance in structure is made consequent upon a free as opposed to a sessile existence. Owing, probably, to the hypoblast actually lining the wall of the body in the Actinozoa, the gastric pouches can only be formed by ingrowths of the hypoblast and mesoblast into the archenteron. There is, however, in the Echinodermata (fig. 52) a large space, the segmentation-cavity, between the archenteron and the body-wall (epiblast now being lined by mesamoeboids). Thus archenteric diverticula can oe directly formed, and, being developed, can surround the archenteron. This method of forming a true body-cavity is also characteristic of all the ccelomatous Metazoa, although it may be greatly modified and abbreviated. The actual formation of the body-cavity in representative examples of the Metazoa has already been briefly described.

Metamerism. - When only a single pair of archenteric diverticula are formed, the animal is, in the true sense of the term, unsegmented. But usually a considerable number of diverticula appear, either directly from the archenteron, as in Amphioxus (fig. 57), or indirectly from the lateral mesoblastic bands, the abbreviated but usual method (p. 56). These forms are termed segmented, and the segments may remain more or less distinct (Chaetopod Worms) or become almost obliterated (Vertebrates).

The question of metameric segmentation is too intricate a one to be here discussed. It must suffice to point out that, while externally unsegmented, many Platyhelminth Worms have a repetition of their internal organs, especially in the case of the gastric diverticula and the generative glands. In the Chsetopoda the body is divided into a large number of mesoblastic somites, and more or fewer of the organs may be implicated in this metamerism. In the great majority of Arthropods the segmentation tends to become obscured - it only affecting the exoskeleton, the appendages, the muscular system, and the nervous system.

The metamerism of the Chordata has many peculiar features, as several important organs are unaffected by it, and others only partially so. The neural plate and notochord always appear very early, and are from the first unsegmented. Whether primitively segmental or not, the nerves would necessarily acquire a serial position if the muscles were segmented. The segmentation of the vertebral column is unquestionably a secondary phenomenon. As Bateson points out in dealing with the ancestry of the Chordata, the segmentation of the gill-slits has been acquired within the group of the Chordata, as nothing resembling them occurs outside it. The liver is from the first a single structure (e.g., Amphioxus upwards), and never shows any indication of having a paired or multiple origin. Although the mesodermal segmentation of Amphioxus is so marked (figs. 5 6, 57), the metamerism of the Chordata is really very partial, and there is insufficient evidence in support of the view that the Chordata were derived from segmented ancestors ; the converse proposition is perhaps more in accordance with the facts. Hubrecht has brought forward numerous arguments in favour of his belief that the Nemertean Worms and the Chordata arose from a common stock. Dohrn and Semper are the leading advocates of the Annelidan ancestry of the Chordata.

Bilateral Symmetry. - Metamerism and bilateral symmetry are the results of the progression of the animal in a determinate direction, and this also induces the development of paired ambulatory appendages and the specialisation of an anterior region or head, and conversely of a passively following region or tail.

It is as a result of the different impressions made upon them, and of their response to these stimuli, that the different regions of the body possess such marked and constant characters.

When an animal is sessile, external influences may act upon it equally in every direction, and in response to these the animal acquires a radial symmetry ; but when an animal progresses in a definite direction, the two sides of the body will be subject to somewhat different conditions from those affecting the extreme anterior extremity; On the development of distinct muscles to assist in progression, the stress of the muscles would probably make the bilateral symmetry more marked. It is also evident that there would accrue a distinct advantage to the organism if the muscles were symmetrically situated and were of comparatively short length, as by this means they could act in concert or in opposition and give considerable power of motion to the animal. This is exactly the condition of the muscular somites (muscle-plates), which have already been described for numerous embryos.

Those Echinodermata which can move in any direction, such as the Starfish, have a radial symmetry, which almost completely masks their fundamental bilateral symmetry as exhibited in the embryo. Almost without exception the remaining Metazoa are entirely bilaterally symmetrical.

A far greater degree of specialisation can be reached in segmented animals, as the serial multiplication of organs gives the necessary material for concentration, as a consideration of the anterior segments of the higher Worms and the concentration and adaption which has taken place in the head and anterior region of Arthropoda will fully demonstrate.

The region in front of the stomodseum usually projects forward as the pre-oral lobe. This? portions of the body, and that immediately surrounding the mouth, are collectively known as the head. From its position, the head is the seat of most of the sense-organs, and of the most specialised portion of the nervous system.

A post-anal extension of the body constitutes the tail; this very rarely exhibits any features of special interest apart from the mechanical function of propulsion, which it sometimes performs.

Paired lobes from the head or sides of the body are usually developed, which are jointed only in Arthropods. The dorsal processes on the head are usually sensory in function ; when ventral cephalic appendages are present they are modified to form masticatory organs (jaws). The paired lateral appendages of the body variously serve for progresssion, prehension, or respiration.

It must be taken as granted that the form of any given embryo is determined by two causes, first by inheritance, and secondly by the special conditions in which it is placed. It is one of the most difficult of embryological problems to distinguish between these two, and to discover whether the larval form has any special phylogenetic significance.

The characteristic larval forms of most groups of animals are now recognised to be of such great importance that they are described in most zoological text-books, and therefore need not be here dealt with.

Fate of the Blastopore. - The fate of the blastopore is so varied as to have led to very different conceptions concerning its real nature, since the blastopore may persist as the mouth or the anus, or as both, or it may form neither.

A. Invertebrates. - Without entering deeply into controversial questions, it may be regarded as a generally received opinion that the blastopore was primitively elongated (see fig. 17). In Peripatus (fig. 69), which is admittedly an unspecialised form, the elongated blastopore becomes constricted in the middle, thus leaving an orifice at each end, one of which persists as the mouth and the other as the anus. Both these orifices communicate with the archenteron, and as the body elongates the apertures become widely separate, and form the terminal openings of the alimentary canal. The ventral or neural aspect of the body thus corresponds with the surface on which the blastopore occurs, the fused lips of the blastopore coinciding with the median ventral line.

As an ingrowth of epiblast usually occurs round the lips of the blastopore, the cavities of the mouth and anus are lined with epiblast. As has been previously mentioned, the oral invagination is termed the stomodseum, and the anal is called the proctodseum. As a rule, the stomodaeum and proctodseum constitute a very small portion of the alimentary canal as compared with that which is formed by the archenteron (mesenteron). In Crustacea, however, the hypoblastic portion of the alimentary canal is, as a rule, relatively very minute (fig. 140).

In Nudibranchs the elongated blastopore closes over from behind forwards, so that only the oral aperture persists (fig. 17). In the Pulmonate Mollusc Paludina it is the anus which remains unclosed, and in most cases when the blastopore persists it does so as the anus.

Fig. 69. - Embryos of Peripatus Capensis. [After Balfour.]

A. Surface view of gastrula with, elongated somewhat constricted blastopore. B. Later embryo in which the sides of the still more elongated blastopore have grown together ; five mesoblastic somites are present. C. Transverse section through the blastopore of the last.

a. anus (proctodseum) ; b. c. body-cavity (coelom) ; bp. blastopore ; ep. epiblast ; hy. hypoblast ; m. mouth in B, mesoblastic somite in C ; me. mesenteron.

Chordata. - The relation of the mouth and the anus of the Chordata to the blastopore is a problem which is at the present time receiving considerable attention.

The belief, however, is gaining ground that the neural aspect of the body in Vertebrates is identical with that of Invertebrates ; in other words, the terms dorsal and ventral have opposite meanings as ordinarily applied in these two groups..

An ancestral form, of the Chordata may be conceived as having been an elongated animal with a mouth and anus which were the persistent terminal orifices of the elongated blastopore. The body was produced in front of the mouth into a pre-oral lobe, but the anus was situated at the extreme hinder end of the animal. The segmented body-cavity was derived from archenteric diverticula, as is now the case in Amphioxus. The nervous system was differentiated from the external skin, and, being derived from a nervous ring round the primitive blastopore, consisted of a ventral plate mainly situated between the mouth and the anus ; the symmetrical halves of which it is composed would result from the junction of the lips of the blastopore. In front of the mouth the neural plate was greatly enlarged in connection with the specialisation of the pre-oral lobe to form the brain, on which the pit-like eyes were situated (fig. 139).

The folding over of the neural plate to form a neural tube greatly diminished the facility of the communication of the archenteron with the exterior. In the larval Amphioxus the archenteron for a long time opens into the posterior end of the neural canal, through what is known as the neurenteric canal (fig. 57), the neural canal itself opening to the exterior by an anterior pore. But the anterior region of the archenteron (pharynx) communicated with the exterior by means of the developing gill-slits ; and it is assumed by Dohrn and others that an anterior pair of gillslits gradually became modified to form the vertebrate mouth. Sedgwick, however, believes that the mouth is homologous all through the Metazoa, and that it always retains its original position at the anterior end of the true primitive blastopore.

Most embryologists consider the anus of Vertebrates to be a new structure, but Sedgwick regards it as the posterior extremity of the primitive blastopore. In the Lamprey, and several Amphibia, the blastopore is stated to remain permanently open, and to persist as the anus. Weldon describes the proctodaeum in the Lizard as arising within the region of the primitive streak. If the second view be established, it follows that, as in many Invertebrates, the anus of the Chordata assumes a secondary position on the opposite, abneural, side of the body to its place of origin, owing to the elongation of the body. This prolongation constitutes the tail of the Chordata, see figs. 98, 99, which illustrate this for the Frog.

It has been further supposed by Cunningham that the neural plate of the primitive Chordata was folded along the median line, so as to form a groove into which the primitive mouth and anus opened. By this time the anterior region of the archenteron was perforated by paired slits, forming the characteristic respiratory pharynx of the group.

The primitive mouth opened into the archenteron near the anterior extremity of the neural plate. The folding over of the latter to form the neural canal would render the former useless, and a pair of gill-slits are supposed to have assumed its function. Cunningham suggests that the infundibulum (see p. 1 10) is the remnant of the primitive mouth, a view which he maintains is supported by the relations of that diverticulum.

The invagination of the neural plate caused the eyes, which appear to have been simple pit-like depressions of the pre-oral lobe, to develop as outgrowths from the anterior region of the brain (fig. 139) ; the relative position of the ganglionic to the retinal layer of the optic vesicle entirely supports this conclusion. An account of the development of the eye will be given later (p. 157). Other sense-organs were developed according to the requirements of the animal.

The limbs of Vertebrates are now usually considered to be specialisations of a primitively continuous lateral ridge or fin.

Accepting the interpretation given above of the homology of the Vertebrate embryo, the following fusions of the embryonic layers must be supposed to occur (see fig. 62).

1. The fusion of the lips of the primitive blastopore, extending from the primitive mouth to the primitive anus, a region which roughly corresponds with the neural plate. Miss Johnson has described a primitive groove and a primitive streak with the fusion of the layers in this region in the Newt.

2. The union of the lips of the blastoderm behind the embryo in telolecithal ova, forming the “primitive streak- of most authors.

3. The junction of the edges of the blastoderm as they unite after extending over the yolk.

Free Larvae. - Embryos may commence a free existence in practically any stage of development, though the age at which an embryo is hatched or born is definite for the species, if not for the group.

Those forms which commence their free existence at an early stage of development possess many larval structures and organs to enable them to hold their own in the struggle for existence. During their further life-history they pass through regular stages of development, which are usually attained by gradual growth ; but in some cases (e.g., Arthropoda) the changes are hurried over during moults of the skin. Speaking generally, alecithal ova are soonest hatched.

The acquirement of food-yolk is associated with a prolongation of pre-natal existence, but the tendency to undergo a metamorphosis still persists. Consequently rudimentary organs occur during development which receive their explanation in the loss of a free larval life, and even moultings of the skin may occur.

In Vertebrates higher than the Amphibia (Amniota) certain foetal appendages are developed, which must now be considered.

Foetal Membranes of Birds. - The following account of the embryonic appendages refers to the Fowl, but doubtless it is equally applicable to other Birds.

Owing to the large amount of yolk present in the ova of Birds the embryonic area is relatively small. At first the germinal disc is flat, but soon the anterior extremity of the embryo is limited by a fold in the area pellucida, which is known as the head-fold, and, as was described on p. 39 (fig. 72), the embryo is gradually constricted off from the yolk, which is henceforth known as the yolk-sac (umbilical vesicle of Mammals).

The middle germinal layer (mesoblast) early splits into two layers ; the outer layer unites with the epiblast to form the somatopleur or body-wall and the inner unites with the hypoblast and constitutes the somatopleur. The space thus produced, and which is surrounded by the mesoblast, is the future body-cavity (coelom), but it is often termed the pleuro-peritoneal cavity, as being the cavity which encloses the lungs and abdominal viscera ; as will be subsequently described, the lungs come to be enclosed in a special portion of the coelom. The splitting of the mesoblast first occurs in the embryonic area, but as the mesoblast extends farther and farther round the yolk, it continues to split, as will be seen in figs. 70-75. Thus when the mesoblast entirely surrounds the yolk-sac (fig. 72, F and G; 7 5, d), the latter really lies within the body- cavity (pleuro-peritoneal cavity) of the embryo. By this time the yolk-sac is greatly reduced in size owing to the absorption of the yolk by the hypoblast and blood-vessels of the area vasculosa, and ultimately it dwindles away.

Fig. 70. - Transverse Section of an Embryo Fowl of Three Days - Incubation. The size of the embryo is exaggerated. [Prom KolUlcer.]

am. amniotic cavity; blh. extension of the pleuroperitoneal cavity outside the embryo ; d. vitelline membrane ; dr. cavity of the mesenteron ; ect. epiblast ; ent. hypoblast ; g. yolk ; mes. border of the splanchnic mesoblast (area vasculosa) ; r. edge of the blastoderm, here consisting only of epiblast and hypoblast ; s. serous or subzonal membrane.

Amnion. - About the twentieth hour of incubation of a Fowl -s egg a semilunar fold of the blastoderm appears in front of the future anterior extremity of the embryo (fig. 33). This fold, which

Fig. 71. - -Details of the Edge of the Mesoblast of a Fowl -s Ovum about the Stage of Fig. 70. [After Duval.]

ep. epiblast ; hy.n. free nuclei in the yolk, which will give rise to the hypoblast of the yolk-sac ; pp. pleuro-peritoneal cavity or coelom ; so.m. somatic mesoblast; sp.m. splanchnic mesoblast ; y. yolk. J4

is a reduplicature of the somatopleur, is the anterior fold of the amnion. Somewhat later a second fold makes its appearance behind the posterior extremity of the embryo ; this unites with the anterior fold through the production of lateral folds, and the embryo lies in a shallow depression bounded by the amniotic fold. The folds now increase in size (fig. 72, A, D, b), and soon unite in the median line above the embryo ; their walls coalesce, and finally break down at the points of apposition, so that the enclosed cavity becomes continuous (figs. 72, c, E, and 79, 2, 3, 4).

Fig. 72. - Diagrams to Illustrate the Development oe the Amnion and Allantois.

[From Bell, after Foster and Balfour.']

In A the embryo ( e ) is being constricted off from the yolk-sac, and the folds of the amnion are to be seen rising up at either end of the embryo, the anterior fold (at) being, the larger ; in B the amniotic folds nearly meet, and in C they have entirely coalesced. In D, which is a litt'e later stage than A, the allantois (al) is budding out from the intestine ; in E, which is a stage corresponding with C, the allantois is seen extending round the embryo. In F the yolk-sac (y) is reduced in size, and in G it is being withdrawn into the body of the embryo. The allantois in F and G is omitted for the sake of sim plicity.

These diagrams only very roughly indicate the relations of the parts. In all the embryo is represented by horizontal shading, the pleuro-peritoneal cavity is dotted, and the yolk-sac has concentric lines. The dotted ^line indicates the vitelline membrane.

The closure of the amniotic orifice by the fusion of the folds takes place from before backwards, till, at the commencement of the third day, a small opening is left over the tail, which then closes over.

The inner membrane of the amnion (amnion proper) thus forms a complete sac round the embryo (figs. 70-83), and the enclosed space is the cavity of the amnion containing the liquor amnii. The outer amniotic membrane (false amnion or serous membrane) lies immediately below the vitelline membrane.

The space between the true and the false amnion, as will be clearly seen on reference to figs. 70-79, is merely an extension of the body-cavity or coelom (pleuro-peritoneal cavity). It is everywhere bounded externally by the somatopleur, and internally by the splanchnopleur, which invests the yolk (fig. 72, B

Fio. 73 . - Formation of the Allantois. Longitudinal section of the posterior extremity of an embryo Fowl of the third day. Osmic acid preparation strongly contracted by the reagent. [From KoLlikerJ] Magnified 150.

al. rudiment of the allantois ; am. amnion ; cl. cloaca ; d. posterior border of the intestino-umbilical orifice ; d' rectum ; dg. splanchnopleur, where the intestinal wall passes round the yolk, thus forming the anterior border of the tail-fold ; s. posterior extremity of embryo.

and e). The body-cavity is thus gradually extending below the yolk-sac at F (fig. 72), the two sides have met, and have quite coalesced in G.

Allantois. - During the formation of the folds of the amnion a sac projects from the splanchnopleur of the hind-gut into the body-cavity. This is the allantois ; it is lined internally with hypoblast (figs. 72-75). The allantois grows rapidly, extending all round the embryo in the space enclosed by the false amnion.

The further history of the allantois in Birds has recently been carefully studied by Duval. He finds that the outer membrane of the allantois fuses with the serous membrane, or, as it is preferable to call it, the subzonal membrane. (The compound tissue thus formed consists of an outer epiblastic epithelium, a middle layer produced by the fusion of the mesoblast of the subzonal membrane (somatic mesoblast) with that of the allantois (splanchnic mesoblast), and an inner epithelium, the hypoblastic lining of the allantois, fig. 74, b).

Instead of remaining, as it were, within the confines of the body-cavity of the embryo, the allantois protrudes beyond the inferior margin of the yolk-sac, of course carrying the subzonal membrane with it (fig. 75, A, b).

The inferior folds of the allantois enclose the albumen and meet one another below the embryo (fig. 75, c). They next considerably overlap each other, and eventually fuse together (fig. 75, d).

Fig. 74. - A. Diagrammatic Loncitudinal Section through the Egg of a Fowl. B. Detail of a Portion of the same at a Time when the Allantois reached the Spot marked x in A. [After Duval . ]

al. cavity of allantois ; alb. albumen ; ali. mesenteron ; al.hy. hypoblastic epithelium of allantois; al.m. mesoblast of allantois; am. cavity of amnion; 6. blood-vessel; emb. embryo; ep. epiblastof outer layer of amnion (serous membrane) ; epiblastic epithelium of inner layer of amnion (amnion proper); mesoblastic layer of latter; sh. egg-shell; sorn. somatic mesoblast of outer layer of amnion ; v.m. vitelline membrahe; •}• point where the mesoblastic tissue of the allantois fuses with that of the serous membrane.

The remaining albumen of the egg is thus enclosed, in a space bounded above by the ventral wall of the yolk-sac, and below by the folds of the allantois. This space is termed by Duval the placental sac. Simple villi grow out from the epiblast lining the placental sac to absorb the contained albumen, the nutriment being conveyed to the embryo by the blood-vessels of both the, yolk-sac and the allantois.

It is interesting to note that at first villi arise from the epiblast of the inferior pole of the yolk-sac (fig. 75, a, b). Later they are developed from that portion of the non-embryonic epiblast which is lined by the allantois ; in other words, from a true chorion (see p. 90).

The cavity of the amnion gradually extends all round the em

Fig. 75. - Diagrams Illustrating the Formation of the Placental Sac in Birds.

[After Duval.]

A. Section of an egg of a Warbler (“ Fauvette -) corresponding to that of a Fowl from the eighth to the tenth day. B. Detail of a portion of the above. C. Ventral portion of an egg of the same, corresponding to that of a Fowl about the fifteenth day. The two allantoic culs-de-sae have come into contact, forming a placental sac with internal villi. D. Diminished placental sac of the same, shortly before hatching.

al. cavity of allantois, the thick line in B-D indicates its hypoblastic epithelium ; al.e., al.i. outer and inner layers of the allantois ; am. amnion ; ep. epiblast of serous or sub-zonal membrane, - the dotted line between the epiblast and the hypoblast of the allantois indicates diagrammatically the distinction between the mesoblast of the serous membrane and that of the allantois ; hy. hypoblast surrounding the yolk, - the folds of the hypoblast enclose biood-vessels which have been developed from the splanchnic mesoblast ; hy.n. free nuclei which will form the vitelline hypoblast ; m. unsplit mesoblast ; p.p. extra-embryonic body-cavity (pleuro-peritoneal cavity); p.s. placental sac ; sh. egg-shell ; sp.m. splanchnic mesoblast ; v. epiblastic villi of placental sac ; v.m. vitelline membrane ; y. yolk.

bryo, but for some time leaves a narrow pedicel surrounding the stalks of the yolk-sac and allantois (fig. 72, G, the latter is omitted in this fig., and figs. 79, 5 ; 83). This pedicel is known in Mammals as the umbilical cord. In Birds it ruptures just before hatching after the withdrawal of the yolk-sac into the body-cavity of the embryo. In Mammals it is only severed after birth.

Foetal Membranes of Reptiles

Our knowledge of the foetal membranes of Reptiles is still very imperfect.

The amnion first appears as a hood covering that anterior portion of the embryo which very early sinks into the yolk-sac. The anterior fold of the amnion consists of both epiblast and somatic mesoblast, and it gradually extends backwardly in conjunction with lateral folds which arise along the sides of the neural plate. The posterior fold of the amnion does not appear to be present.

The allantois probably resembles that of Birds.

Haacke has shown that in the Lizard Trachydosaurus asper the egg-shell is absent except for a small disc-shaped rudiment which lies between the yolk-sac and the uterus ; thus the embryo is readily seen through the thin walls of the uterus and the transparent embryonic membranes. This Lizard is viviparous, and the vascular wall of the yolk-sac is only separated from the special capillary network of the uterine vessels, which is concerned in the nutrition of the embryo, by the porous and friable rudiment of the egg-shell.

Foetal Membranes of Mammals

The early stages in the development of the embryo in Mammals closely resemble those of Birds ; but there are a few important differences in the nature of the foetal membranes. The differences are mainly due in Mammals higher than the Monotremes to the absence of an egg-shell with its membranes, and of the albumen and yolk. The ovum is merely protected by the zona radiata (zona pellucida), within which a delicate membrane has been observed (fig. 5).

The hollow yolk-sac or blastodermic vesicle grows rapidly ; being distended by a contained fluid, the zona becomes very thin and early disappears. As has previously been mentioned (p. 45), the germinal area alone of the oosperm possesses the three germinal layers ; the remaining portion of the upper half of the oosperm is lined with epiblast and primitive hypoblast, whereas the lower half of the blastodermic vesicle is composed solely of epiblast (fig. 42).

Simple non- vascular villi, which serve to attach the embryo to the walls of the uterus, usually project from the epiblast of the blastodermic vesicle (subzonal membrane). In the Babbit they only occur on that area of the epiblast under which the mesoblast will not extend (figs. 77, 78), with the exception of a horse-shoe shaped patch which early makes its appearance in the region of the future placenta, and with which it shortly becomes identified (eg. 76.pi).

The following account of the development of the amnion is taken from Van Beneden and Julin -s recent researches on the development of the Rabbit.

Pro-amnion. - The mesoblast (fig. 76, a.v) extends for some distance from the embryo in every direction, except immediately around the head ; but the two limbs of mesoblast which bound this emargination gradually extend round some distance in front of the head and eventually unite (fig. 76). Thus it comes about that there is a nearly circular area in front of the head in which the blastoderm consists of epiblast and hypoblast only.

This area early sinks into the cavity of the blastodermic vesicle, and the anterior extremity of the embryo projects into this depression, which Yan Beneden and Julin term the pro-amnion (figs. 76-78,

Amnion. - Very slightly later the true amnion is developed, but only over the posterior end of the embryo (figs. 76, 77). It rapidly grows forwards until it comes in contact with the raised anterior rim or fold of the pro-amnion, with which it fuses. The cavity of the amnion coalesces with the space (extra-embryonic pleuro-peritoneal cavity) resulting from the splitting of the mesoblast, which now extends in front of the embryo and the proamnion.

Fig. 76. - Diagrammatic Dorsal View of an Embryo Rabbit with its Membranes at thf. Stage of Nine Somites. \ Modified from Van Beneden and Julin.]

al. allantois, showing from behind the tail fold of the embryo ; am. anterior border of true amnion ; a.v. area vasculosa, the outer border of which indicates the farthest extension of the mesoblast ; bl. blastoderm, here consisting only of epiblast and hypoblast ; o.m.v. omphalomesenteric or vitelline veins ; proamnion ; pi. non-vascular epiblastic villi of the future placenta; s.t. sinus terminalis.

In process of time the pro-amnion gradually atrophies, and the true amnion correspondingly advances forwards.

It is now generally admitted that the amnion was primitively caused by the embryo sinking into the yolk-sac by its own weight. The protection to the embryo by the formation round it of what is virtually a water-sac resulted in the precocious development of the amnion before the embryo in its ontogeny had any appreciable weight.

The pro-amnion probably originated from a similar bearing-down of the heaviest (anterior) end of the embryo, when .the blastoderm of that region was still diploblastic (two-layered). The pro-amnion is, in fact, an exaggeration of the head-fold.

Fig. 77. - Diagrammatic Median Vertical Longitudinal Sections through the Embryo Rabbit. [ After Van Beneden and Julin .]

A. Section through embryo of fig. 76. B. Section through embryo of eleven days. al. allantois; am. amnion; anterior median plate of mesoblast, formed by the junction of the anterior horns of the area opaca ; area placentalis ; a.v. area vasculosa ; ch. chorion; ece. coelom of embryo; cce f . extra embryonic portion of the body-cavity ; ep. epiblast ; hy. hypoblast ; m. unsplit mesoblast ; o.a. orifice of amnion; pi. placenta; pro.a. proamnion; s.t. sinus terminalis ; v. epiblastic villi of blastodermic vesicle.

Van Beneden and Julin affirm that it not only occurs in Rodents, but also in Bats and the Dog, and that it probably exists for a short period in the Fowl and in Lizards.

Fig. 78. - Fietal Envelopes of a Rabbit Embryo. [ From Minot,

after Van Beneden and Julin.] Later stage than fig. 77, B. The amnion has become fused with the blastoderm in front of the embryo, and its cavity is therefore continuous with the extra-embryonic portion of the bodycavity in front of the embryo.

Al. allantois ; am. amnion ; am'. portion of the amnion united with the walls of the allantois ;, area placentalis; Av. area vasculosa; Ch. chorion; Coe. coelom or body-cavity; coe". extra-embryonic portion of the body-cavity ; Coel. anterior portion of the same, produced by the fusion of the cavity of the amnion with that of the anterior portion of the area opaca ; Ec. epiblast ; En. alimentary canal of the embryo; Ent. hypoblast; PI. placenta; pro. A. proamnion ; T. sinus terminalis-; V. villi of blastodermic vesicle ; Y. cavity of blastodermic vesicle.

It would appear, therefore, that the pro-amnion is a structure which is comnlon to a greater or less extent to the Sauropsida and Mammalia.

The anterior fold of the true amnion is certainly absent in the Babbit, and this may prove to be the case for Mammals generally, now attention has been drawn to the question. At all events, the posterior fold of the amnion is always well developed.

By this time the partially vascular yolk-sac has gradually diminished in size; and the vascular allantois is greatly increasing in size and importance, and is functionally replacing the yolk-sac.

Allantois. - The Mammalian allantois has a similar origin to that in Birds (figs. 77, 79). It extends to a greater or less extent between the amnion and the serous membrane or subzonal membrane.

The outer membrane of the highly vascular allantois fuses, as in Birds, with the subzonal membrane, the villi of which become vascular and usually grow more complex. The compound membrane thus formed is known as the chorion. That portion of the chorion which enters into immediate connection with the uterus of the mother constitutes the foetal portion of the placenta.

As will be shown later (p. 259), the proximal portion of the stalk (urachus) of the allantois persists as the urinary bladder, and it is generally admitted that the urinary bladder (urocyst) of Amphibia is a homologous organ with that of the Amniota. It is thus a fair assumption to make that the allantois is merely the precociously developed urinary bladder.

In the lower Yertebrates the egg is usually laid in water, and the larva is, as a rule, early hatched, respiration being effected by gills situated on the gill-arches.

In Alytes and Notodelphis ovipara and some other Anura, large external gills are developed while the embryo is still within the egg-covering, their function apparently being to give increased facility for respiration to the unhatched young. A similar condition also occurs in some Elasmobranchs.

Certain Anura, however, have such an abbreviated larval existence that the young are hatched as small Frogs, and in some of these the external gills atrophy early (Pipa americana), or are said to be entirely absent (Rhinoderma darwinii, Nototrema marsupiatum). In Pipa the long tail of the tadpole functions as a respiratory organ [Peters], and the same holds good for Hylodes. Boulenger finds that the abdomen of the just-hatched Rana opisthodon is provided with a lateral series of symmetrical folds, which probably have a respiratory function.

The abo vo facts tend to show that some Frogs are losing their ancestral larval breathing organs, and are utilising other organs for respiratory purposes ; and it is very significant that this occurs amongst those Frogs which do not deposit their eggs in water. It is then not difficult to imagine that some primitive Amphibian which had acquired an increase of food-yolk (as a few recent Anura have done) would find in the urinary bladder an organ which could be pressed into the service of aerial respiration.

If we may assume that some such Amphibian was the ancestor of the Amniota, we have a clue to the significance of the total absence of even rudimentary gill-filaments on the gill arches of even the youngest embryos of the less specialised Amniota in

Fig. 79. - Five Diagrammatic Figures Illustrating the Formation of the Fcetal Membranes of a Mammal. [From Kolliker. J

In 1, 2, 3, 4, the embryo is represented in longitudinal section.

1. Oosperm with zona pellucida, blastodermic vesicle, and embryonic area. 2. Oosperm with commencing formation of umbilical vesicle and amnion. 3. Oosperm with amnion about to close and commencing allantois. 4. Oosperm with villous subzonal membrane, larger allantois, and mouth and anus. 5. Oosperm in which the vascular mesoblast of the allantois has extended round the inner surface of the subzonal membrane, and united with it to form the chorion ; the cavity of the allantois is aborted. The yolk-sac (umbilical vesicle) has greatly diminished. The large amniotic cavity surrounds the umbilical cord. This fig. represents an early human ovum.

a. epiblast of embryo ; a'. epiblast of non-embryonic part of the blastodermic vesicle ; ah. cavity of the amnion ; al. allantois ; am. amnion ; as. amniotic sheath round the umbilical cord ; ch. chorion ; ch.z. villi of chorion ; d. zona pellucida (radiata) ; d'. processes of zona ; dd. embryonic hypoblast ; df. area vasculosa ; dg. stalk of yolk-sac; ds. yolk-sac (umbilical vesicle); e. embryo ; hh. pericardial cavity ; i. non-embryonic hypoblast ; kh. cavity of the blastodermic vesicle, which practically is equivalent to the yolk-sac ; ks. head-fold of amnion ; m. embryonic, m' non-embryonic, mesoblast ; r. space between chorion and amnion containing albuminous fluid ; sh. subzonal (serous) membrane ; st. sinus terminalis ; sz. subzonal villi ; vl. ventral body-wall in the region of the heart.

the supposition that the loss of larval gills was a pre-amniote character. This was rendered possible before the lungs were functional in ontology by the acquisition of an accessory respiratory organ ; in this case it was the thin-walled vascular urinary bladder. The adoption of this organ for respiratory purposes causes it to grow enormously in size, and at the same time to appear earlier. Hence the great development it now attains.

It has just been shown that in Birds the epiblast which underlies the yolk-sac is produced into villi (fig. 75, b, v), which absorb the nutritive albumen before the allantoic villi are developed. The same also occurs in the lower Mammalia.

In the Virginian Opossum (Didelphys), according to Osborn, when the allantois is still very small, the yolk-sac is provided with simple vascular villi (fig. 80, v), which, in addition to serving to attach the embryo to the uterine wall, are undoubtedly nutritive in function. In these Mammalia there is no albumen to feed

Fig. 80. - Diagram of the Fcetal Membranes of the Virginian Opossum. \_After Osborn .J

Two villi are shown greatly enlarged. The processes of the cells, which have been exaggerated, doubtless correspond to the pseudopodia described by Caldwell.

al. allantois ; am. amnion ; st. sinus terminalis ; sz. subzonal membrane ; v. villi on the subzonal membrane in the region of the yolk-sac ; ys. yolksac. The vascular splanchnopleur (hypoblast and mesoblast) is indicated by the black line.

upon, but better nutriment can be directly obtained by osmosis from the mother.

Caldwell has shown that in the Native Bear (Phascolarctos cinereus) (fig. 81) the inferior non- vascular moiety of the yolk-sac is, even up to a comparatively late period, surrounded only by hypoblast and the non-embryonic epiblast (subzonal membrane). The cells of the latter send out pseudopodia (fig. 81, amb), which fit in between the cells of the uterine epithelium. Although the allantois is larger than in the preceding form, and comes into contact with the subzonal membrane, no villi are formed by it ; in other words, in the Marsupials the true chorion, if present, is rudimentary, and, so far as is known, never develops villi. The previous researches of Owen point to the same conclusion.

Prom the nature of the case, no adhesion occurs between the embryo and the parent in the Prototheria, any more than in Sauropsida. In the Metatheria a very slight connection does occur, but in this union the subzonal membrane surrounding the yolk-sac alone takes part. As the latter was the sole nutritive organ of the embryos of the earlier Mammals, it would probably but slowly part with this function.

Ryder has suggested that the degeneracy of the yolk in the Mammalian oosperm

-may be due to the development of the so-called uterine milk from the uterine glands, and it subsequently completely disappeared in consequence of the perfectly parasitic connection temporarily subsisting between the mother and the embryo. (The latter supposition was first put forward by Balfour.) At this stage of evolution the allantois was respiratory, -as it practically is in the Sauropsida, Monotremes, and , Marsupials, and the yolk-sac was becoming less nutritive in function.

As the allantois is used in Birds to absorb the albumen, so in the higher Mammals (Eutheria) it develops villi where it is fused with the subzonal membrane, and forms the chorion.

Fig. 8i. - Diagram of the Fcetal Membranes of the Native Bear. \After Caldwell .]

al: allantois; am. amnion ; amb. amoeboid processes of the subzonal epiblast in the non-vascular region of the yolk-sac ; hy. hypoblast of the non-vascular region of the yolk-sac; s.t. sinus terminalis; s.z. subzonal membrane ; y.s. yolk-sac. The black line indicates the vascular splanchnopleur (hypoblast and mesoblast). A greatly magnified portion of the ventral wall of the yolk-sac is also given.

The term chorion is now limited to those areas of the subzonal ^membrane to which the yolk-sac or the allantois are attached. Balfour distinguished the former of these as the false and the latter as the true chorion. In the Babbit (fig. 82) the false chorion is very large, and the true (or placental) chorion relatively small ; but in most Mammals the true chorion has a much greater ex^ tension.

It is possibly owing to the large size of the yolk-sac that the allantois, forms such a small chorion in the Babbit. There is a remarkably close resemblance between the general disposition and structure of the foetal membranes in the Babbit (%s. 78, 82) and some Marsupials (figs. 80, 81). In both, the epiblast (subzonal membrane) of the yolk-sac (blastodermic vesicle) gives rise to non' vascular villi only in the region where the mesoblast has not extended. The allantois also unites with the subzonal membrane above the embryo to a small extent ; but in the Rabbit vascular villi are developed at this spot, which thus form a true placenta.

The epiblast of the blastodermic vesicle appears to give rise to villi in other Mammals, but more precise information is required on this point.

The nature and position of the villi of the chorion vary considerably. The villi fit into depressions of crypts of the uterine wall, the conjoint structure being known as the placenta.

The placenta of the Rodentia, Insectivora, and Chiroptera is usually dorsally situated and discoidal, as in the Rabbit, and ife

Fig. 82. - Diagrammatic Longitudinal Section of Oosperm of Rabbit at an Advanced Stage of Pregnancy. [From Kolliker after B ischoff.]

' a. amnion ; al. allantois with its bloodvessels ; c. embryo; ds. yolk-sac; ed, ed', ed". hypoblastic epithelium of the .yolk-sac ana its stalk (umbilical vesicle and cord) ; fd. vascular mesoblastic membrane of the umbilical cord and vesicle; pi. placental villi formed by the allantoig and subzonal membrane;*r. space filled with fluid between the amnion, the allantois,- and the yolk-sac ; st. sinus terminalis (marginal vitelline blood-vessel) ; u. urachus or stalk of the allantois.

^co-extensive with the area of contact between the allantois and the ^subzonal membrane. In these forms the yolk-sac is in contact with the larger portion of the subzonal membrane.

In Edentata the placenta may be discoidal (Loricata), or domeshaped (Pilosa), or zonary (Tubulidentata), that is, occupying a •broad band round the chorion, leaving the ends free from villi, or diffuse (Squamata).

In the Dog the large vascular yolk-sac does not fuse with the subzonal membrane. The allantois first grows out on the dorsal side of the embryo, where, coalescing with the subzonal membrane, it forms an at first discoidal placenta. The villi soon extend, so as to form, a zonary placenta. The zonary placenta is found in the Carnivora, llyrax, and Elephas.

The extension of the placenta over the whole of the chorion results in what is termed a diffused placenta ; this is characteristic of the Perissodactyla, the Suina, the Tragulina, the Tylopoda, the Sirenia, the Cetacea, the Lemuroidea.

The collection of the villi into groups constitutes what is known as a cotyledonary placenta. This variety is confined to the Pecora. In the Giraffe, the placenta is partly diffused and partly cotyledonary. Weldon finds that in the Pour-horned Antelope (Tetraceros) the whole surface of the chorion is thrown into vascular ridges, exactly as in the Pig, and the cotyledons are very few in number (twenty to thirty), other Antelopes having sixty or more. The Bovidse possess a large number of cotyledons, while the Cervidse have only a very few. In Moschus, however, the placenta is finely folded, cotyledons being absent.

In the Anthropoidea, the villi are at first diffuse, but ultimately they are restricted to the ventral surface, forming a secondary discoidal placenta (metadiscoidal).

The simplest kind of placenta is one in which the papilla-like villi of the chorion fit into corresponding depressions in the uterus. The villi are ranged in irregular ridges in the Pig. In such forms the chorion can be withdrawn at birth from the placenta; in other words, the placenta is non-deciduate.

The following animals have a non-deciduate placenta : - Artiodactyla, Perissodactyla, Sirenia, Cetacea, Lemuroidea, and some Edentata (Squamata). But in some of these the villi are more or less branched and complicated ; and in many of the Pecora this interlocking is so close that the parts of the epithelium of the maternal cotyledons may be carried away at birth.

In all the other Eutheria the foetal villi are so intimately connected with the uterine wall, that at birth a greater or less portion is brought away with the allantois (after-birth). This form of placenta is known as the deciduate.

The uterus merely develops short tubular crypts to surround the foetal villi in the case of those Mammals with a simple nondeciduate placenta. But in those with a deciduate placenta the wall of the uterus undergoes varied structural modifications, which reach their extreme form in the Anthropoidea, where the foetal villi are immersed in large uterine blood-sinuses.

Very shortly after the human ovum has entered the uterus, the walls of the latter grow round and incapsulate it (fig. 83). The reflected portion of the uterus is called the decidua reflexa. That portion of the wall to which the embryo is attached is known as the decidua serotina, the decidua vera being the remaining surface of the uterus. All these structures are cast off in the act of birth.

The decidua reflexa is more or less developed in a few other Mammals, e.g., Seals, and some Insectivora.

Inversion of Germinal Layers in Rodents. - A peculiar inversion of the germinal layers in the blastoderm of the Guinea-pig was first described by Bischoff, and later confirmed by Hensen and Schaffer. Four papers were simultaneously published at the end of the year 1882, in each of which there was practically an identical solution

Fig. 83. - Diagrammatic Section of Pregnant Homan Uterus, with Contained Fcetus. {From Huxley after Longet .]

al. allantoic stalk (urachus); am. amnion; c. cervix uteri ; ch. chorion; dr. decidua reflexa ; ds. decidua serotina ; du. decidua vera ; l. Fallopian tube (oviduct) ; nb. umbilical vesicle or yolk-sac ; 2. foetal villi of the true placenta ; 2'. villi of the non-placental part of the chorion.

The portion of the uterine wall to which the embryo is attached is the decidua serotina ; that portion which grows round the embryo is the decidua reflexa, while the general wall of the uterus, not related to the embryo, is the decidua vera.

of this difficult problem. The forms studied were the Field-vole (Arvicola arvalis) by Kupffer, the House-mouse (Mus musculus) by Selenka, the Guinea-pig (Cavia cobaya) by Hensen, and the House-mouse, the Rat (Mus decumanus), and the Guineapig by Fraser. Slightly later, fresh light was thrown on the subject by Spee, and lastly Heape -s researches on the Mole (Talpa europea) have supplied additional information.

The explanation of the phenomenon is briefly as follows. As has already been described, the solid mass of inner-layer cells, attached to one pole of the blastodermic vesicle in the Rabbit (fig. 39, b, c ), flattens out to form the germinal disc (fig. 39, d).

In the Mole the primitive inversion of the blastoderm is retained slightly longer, the embryonic epiblast forming a cup-like depression at one pole of the blastodermic vesicle ; the secondary cavity thus formed being filled with loose cells of epiblastic origin. The whole is roofed over by a layer of covering cells, which is continuous with the outer wall of the blastodermic vesicle (compare fig. 45, b). Later, in this Insectivore the blastoderm becomes flattened out, and development proceeds much as in the Rabbit.

In the Field- vole the ovum forms a normal blastodermic vesicle, with a blastoderm consisting of epiblast and primitive hypoblast (fig. 41). The layer of covering cells which overlies the embryonic epiblast is the seat of an early and rapid proliferation (fig. 83V a), thus forming a mass of cells which pushes the blastoderm before it (fig. 83*, b). The embryo is developed from the centre of the germinal area, the folds of the amnion arising between the embryo and the covering cells (fig. 83*, c).

In the House-mouse and Rat the blastoderm is pushed for a considerable distance within the blastodermic vesicle by the proliferating epiblast (fig. 83*, e). Subsequently an elongated cavity appears within the latter, extending along the whole length of

Fig. 83*. - Diagrams Illustrating the Inversion of the Germinal Layers in the Blastodermic Vesicle of Rodents.

A-C. Field-vole [after Kupffer ]. D. House-mouse [ after SelenJca ]. E-H. Rat [after Fraser ]. None of the figures are drawn to scale.

a. commencement of the folds of the amnion ; all. allantois ; b.v. blood-vessel of uterine wall ; c.c. covering cells which primitively overlie the blastoderm, and which serve to connect the future placental- pole of the blastodermic vesicle with the wall of the uterus ; e.a. embryonic area of blastoderm ; emb. embryo ; e.p. embryonic epiblast ; ep'. non-embryonic epiblast, or epiblast of blastodermic vesicle ; f.a. false amnion or serous membrane; hy. hypoblast; m. mesoblast ; n.a. neuramniotic cavity (amni otic cavity); s.c. secondary cavity; y.s. cavity of yolk-sac or blastodermic vesicle. In the Rat (E-H) the wall of the blastodermic vesicle consists of two layers, epiblast and hypoblast ; only the former is shown in the diagrams. The notch above the line pointing from m in H indicates the neurenteric canal, and marks the posterior end of the embryo.

the previously solid plug of epiblast cells. This cavity clearly corresponds to the hollow simple cup-like invagination of the blastoderm of the Mole and Field-vole. The germinal disc occupies the bottom of the depression, and the embryo develops on the upper surface of the secondary cavity (fig. 83*, h, emb) ; thus, to borrow an illustration, at a certain stage (fig. 83*, c, f) the embryo bears the same relation to the secondary cavity that the embryo Fowl does to the cavity of the amnion at an early stage in the formation of the amniotic folds (fig. 72, a, b). Whether it is rectified (Talpa) or not (Arvicola, Mus), the body of the embryo always lies morphologically outside the blastodermic vesicle.

The primitive elongated secondary cavity of the House-mouse and Rat soon becomes constricted into two vesicles, one of which occupies the fundus of the involution, while the other lies in its stalk (fig. 83*, f, g). The former has been named by Fraser the neur-amniotic cavity, as it is from the walls of this vesicle alone that the embryo is formed. This vesicle is merely the isolated extremity of the primitive secondary cavity ; its wall is composed of an inner layer of epiblast and an outer layer of primitive hypoblast, and the dorsal surface of the embryo consequently projects into its central cavity. In other words, it is the cavity of the amnion of more normal embryos (fig. 79, 3-5, ah).

The second vesicle encloses what Fraser terms the false amnion cavity. The epiblastic epithelium lining it is the exact equivalent of the false amnion or serous membrane of other Amniotes.

During further development these two vesicles become separated by a considerable space from one another. The mesoblast, which has by this time made its appearance in the embryonic area, extends into this “interamniotic space,- and the allantois also penetrates into it as an, at first, solid bud of cells (fig. 83*, H, all). The interamniotic space into which the mesoblast and allantois immigrate is simply the extra-embryonic body-cavity (pleuro-peritoneal cavity) of other forms (figs. 72, 77, 78, coe).

The last term of the series is found in the Guinea-pig, in which Rodent the neuramniotic cavity, with its embryonic area, appears to be precociously separated from the upper pole of the blastodermic vesicle, so as to form a vesicle at the opposite pole. The neur-amniotic vesicle is thus a hollow ball composed of two layers of cells, the outer layer being the primitive hypoblast and the inner layer the epiblast. There is a thick ingrowth or plug (“ Trager -) of epiblast cells at the upper pole, as in the House-mouse (fig. 83*, d).

Summary of Evolution of Foetal Membranes. - Food-yolk is stored up in the primitive hypoblast of most Vertebrates, sometimes to an enormous extent. In the latter case the embryo is, as it were, pinched off from the large yolk-sac.

During its development the embryo digests and absorbs the yolk by means of the surrounding hypoblast and the vascular splanchnopleur. In the case of a few Elasmobranchs the vascular yolk also obtains nutriment directly from the blood-vessels of the enlarged oviduct (uterus) of the mother, prominences from the yolk-sac fitting into depressions of the oviduct. In the Teleost Anableps the vascular yolk-sac is provided with villi, which absorb nutriment from the fluid secreted by the walls of the dilated ovarian chamber, within which the embryos are developed [Wyman].

T. J. Parker finds in Mustelus antarcticus that the pregnant oviduct was subdivided into five to eight compartments, each containing one embryo. The wall of each compartment can be resolved into two *â– layers : an outer highly vascular membrane (pseudo-chorion), derived, from the oviduct ; and an inner cuticular non-vascular layer, secreted by the former. As the enclosed cavity is tense with a fluid giving the reactions of the amniotic fluid, as generally understood, he proposes to call the latter membrane the pseudamnion.

In Birds, simple villi develop from the yolk-sac for the purpose of absorbing the albumen.

When the ancestors of the Metatheria (Didelphia) and Eutheria (Monodelphia) were ceasing to deposit their eggs, and were retaining the by-this-time shell-less ova within the oviduct, the ova were placed in a most favourable condition for obtaining supplemental nutriment. The vascular yolk-sac would readily become slightly attached to the wall of the oviduct, as in some Elasmobranchs and Lizards (Trachydosaurus and Cyclodus [Haacke]).

The nutriment (blood of the oviduct or uterus, and probably the secretion of the uterine glands) thus at the disposal of the ovum was more easily assimilated than the yolk; and it is not surprising that the yolk-sac gradually lost its yolk, and that the embryo became entirely dependent upon the maternal bloodvessels. The yolkless yolk-sac of Mammals is known as the blastodermic vesicle.

The blastodermic vesicle was primitively the only means of connection between the embryo and the parent, as it still is in the Metatheria, and at first is in the embryos of the Eutheria.

By this time the allantois, from being an almost purely respiratory organ, became attached to the serous or subzonal membrane, and assumed a nutritive function. In Birds (and probably in Reptiles), the allantoic villi also absorb the albumen which lies within the egg-shell. In the Eutheria, the egg-shell being absent, the villi enter into direct union with the uterine wall. As the allantois became more closely attached to the uterus, it gradually usurped the functions of the yolk-sac, and eventually entirely superseded it.

The allantoic villi are collectively termed the placenta, and distinct lines of specialisation in the disposition of the villi and structure of the placenta can be traced in the Eutheria, the main object to be gained being the increase in the facility for transfusion between the maternal and foetal fluids. The result is that in the higher forms the villi become more complex, and instead of being readily withdrawn from the uterine crypts at birth, they fuse with the uterine wall, and thus form a deciduate, as opposed to a non-deciduate placenta.

The complex foetal membranes of the higher Eutheria are evidently the result of the gradual differentiation of pre-existing structures.

Amnion of Insects. - The Insects are characterised by possessing an embryonic protective membrane, which is termed the amnion. It consists of a reduplicature of the epiblast, which extends over the ventral (neural) aspect of the body and encloses all the appendages.

The two amniotic folds unite and fuse in the median ventral line below the developing embryo, and the two membranes thus formed separate and constitute a double covering for the embryo, as in the case of the Amniota.

In the Insects, the two folds of the amnion are purely epiblastic in origin, but they may conveniently receive the same relative names as those of the Amniota, the outer one being called the serous membrane, and the membrane next to the embryo is termed the amnion proper.

If they have not previously disappeared, the amniotic membranes are either absorbed or cast off at hatching.

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Haddon 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

Cite this page: Hill, M.A. (2019, July 21) Embryology Book - An Introduction to the Study of Embryology 4. Retrieved from

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G