Book - Vertebrate Zoology (1928) 16

CHAPTER XVI

THE EMBRYONIC MEMBRANES

The Yolk-sac. — In those forms in which the quantity of yolk contained in the egg is large, the embryo is formed from a blastoderm on the surface of the yolk and does not wait for the latter to be enclosed. So it comes about that the yolk is not situated within the embryo, as, for example, it is in the frog ; indeed, in the chick it would be manifestly impossible. In the heavily-yolked forms, then, the yolk is outside the embryo, and it becomes surrounded by a layer of cells which are endodermal and continuous with those of the gut-wall inside the embryo. The yolk then finds itself inside a " yolk-sac," which may be regarded as temporarily extra-embryonic gut. This sac carries a layer of mesoderm outside its (endodermal) wall, and blood-vessels passing between the mesoderm and endoderm absorb the yolk (which has been digested) and convey it into the embryo. Chief among these vessels are the vitelline arteries and veins. Indeed, in most groups of verte- brates, the wall of the yolk-sac is the site of origin of the blood in the form of blood-islands.

In the fish, the function of the yolk-sac circulation is not only to convey digested yolk, but also to oxygenate the blood in its many capillaries, at the early stages of development before the gills have become functional.

The yolk-sac reaches the height of its development in reptiles and birds ; and in the Monotremes which, although mammals are oviparous, yolk is present and the yolk-sac is large. What yolk there is in the egg of the Marsupials is extruded, and the egg of the Placental mammals contains no yolk. Nevertheless, in both the last-mentioned groups a yolk-sac is present although it contains no yolk.

In several groups of vertebrates, the yolk-sac may come to bear interesting relations to the wall of the oviduct, with which it is in contact if the egg is not laid but undergoes development within the body of the mother. The blood-vessels of the yolk-sac are in these cases able to absorb substances from the circulation of the mother (by diffusion), and such an organ of physiological communication between mother and embryo is a placenta. It is necessary to specify the organ which forms the placenta, and a placenta derived from the yolk-sac is called an omphaloidean placenta, to distinguish it from the allantoic placenta which is formed by the allantois.

An omphaloidean placenta is present in the dogfish Mustelus, and in the reptile Chalcides ; it is the chief nutritive organ in the embryonic development of the Marsupials (in fact, it is the only form of placenta in all except Perameles which in addition has an allantoic placenta), and in the " Placental " mammals it arises early and disappears later.

As development proceeds, and the quantity of yolk is reduced, the size of the yolk-sac decreases and finally it is withdrawn into the body through the umbilical stalk.

The Allantois. — The allantois occurs in reptiles, birds, and mammals, and attains its greatest development in the latter. It develops as an outgrowth from the hind part of the gut, and is an endodermal sac covered with mesoderm in which blood- vessels run. In amphibia it is represented by the (allantoic) bladder. In reptiles, birds, and Monotremes, the allantois functions as a respiratory and excretory organ, for which it is well fitted since the excretory ducts open into its base, and its distal portion is spread out close beneath the (porous) shell. In the reptile Chalcides, the Marsupial Perameles, and the Placental mammals, the allantois enters into relations with the wall of the oviduct (or uterus) and forms the allantoic placenta. Its function is then nutritive as well as respiratory and excretory. It is easy to see how this may have occurred in evolution by the retention of the egg within the oviduct and the disappearance of the shell. It is necessary to mention this last proviso because in some forms the egg is not laid ; it undergoes development in the oviduct but does not lose the shell. This condition, which occurs in the viper, is called ovo-viviparous.

In the Placental mammals, the allantois relieves the yolk-sac in the formation of the placenta, and the higher the order of mammals the earlier does this happen. Indeed, in the highest of all, the Primates (including man), the mesoderm of the allantoic stalk appears from the beginning (the " body-stalk "), and the endodermal allantois grows into it later. In these animals the allantoic blood-vessels (the umbilical arteries and veins) are ready at a very early stage to transport to and from the embryo, which increases the efficiency of the placenta. The blood-vessels of the allantois are usually covered by the outermost layer of extra-embryonic ectoderm, known as the chorion in reptiles and birds, and the trophoblast in mammals.

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Fig. 116. — Diagram of the relations of the embryonic membranes in the human embryo. (From Jenkinson, after Graf Spee.) The embryo, developed in the floor of its amniotic cavity (amc) is attached to the trophoblast by the mesodermal body-stalk (bst), into which the allantois is beginning to grow ; bv, blood-vessels round the wall of the yolk-sac (ys) B, transverse section through the body-stalk in the plane indicated.

The Allantoic Placenta. — The blood of the mother and that of the embryo are never in direct communication. The passage of foodstuffs, excretory and respiratory substances must therefore take place by diffusion through the membranes. The efficiency of the placenta is conditioned by the area of mutual contact between the maternal and embryonic circulations, and by the thickness and number of the intervening membranes. The area of contact can be increased by throwing the surfaces of the maternal and embryonic tissues into folds ; and the intervening membranes can be decreased by removal or erosion of certain of the layers of the uterus. Four grades of structure and corresponding efficiency can be seen in the mammals, which will now be taken in order.

(i) The embryonic and maternal surfaces are flat and un- folded ; the area of contact is therefore small. However, the trophoblast (embryonic outermost layer) disappears, and the uterine epithelium (innermost maternal layer) becomes syncytial and contains blood-vessels. Substances therefore have to pass through the wall of the maternal capillaries, through the uterine epithelium, across the intervening space and through the wall of the embryonic capillaries. This type of placenta occurs in Perameles, the only Marsupial to possess an allantoic placenta at all. It was probably present in the ancestors of the Marsupials, and has been lost in the other living Marsupials.

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Fig. 117. — Section through a part of the allantoic placenta of Perameles : embryonic tissue on the left, maternal on the right. (After Hill.) al, allantois ; eb, embryonic blood-vessels ; et)i, embryonic mesoderm ; mb, maternal blood-vessels ; mc, maternal connective tissue ; ue, uterine epithelium (which has become syncytial).

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Fig. 118. — Section through a part of the allantoic placenta of the cow; embryonic tissue to the left, maternal to the right. The trophoblast (t) is produced into villi (v), which fit loosely into crypts (c) in the uterine wall, the epithelium of which (ue) persists ; al, allantois ; eb, embryonic blood-vessel ; ec, embryonic connective tissue ; mb, maternal blood-vessel ; mc, maternal connective tissue ; ug, glands in the wall of the uterus.

(ii) The embryonic and maternal tissues are thrown into folds ; embryonic " fingers " or villi fitting into corresponding crypts in the uterine wall. In the pig, villi are distributed all over the trophoblast, but in the cow they are grouped together in clumps forming cotyledons. The uterine epithelium per- sists. Substances therefore must diffuse through the wall of the maternal capillaries, connective tissue (uterine epithelium), trophoblast, and the wall of the embryonic capillaries.

(iii) The epithelium of the uterus disappears, and the underlying maternal connective tissue is invaded by the developing villi of the trophoblast, so that the latter comes into contact with the walls of the maternal capillaries. Sub- stances have only to pass through the wall of the maternal capillaries, the trophoblast, and the wall of the embryonic capillaries in order to diffuse through. This type of placenta occurs in carnivora (cat and dog), and is restricted to a zone of the trophoblast, whence its name zonary.

(iv) The epithelium of the uterus is removed, but the underlying connective tissue is not invaded as in the carnivores ; instead the trophoblast is very much thickened and then hollowed out here and there to form lacunae. The remaining projections from the trophoblast are called pseudovilli to distinguish them from the true villi which are definite out- growths. The maternal blood-vessels are " tapped " by the very thorough erosion of the uterine wall, and the blood flows out of them and into the lacunae in the trophoblast. The pseudovilli are therefore bathed in the blood of the mother, and the substances have only to pass through the trophoblast and the wall of the embryonic capillaries to enter into the embryonic circulation. This is the highest type of placenta, and it is found in the rabbit, mouse, bat, shrew, hedgehog, mole, Tarsius, monkey, and man. It is interesting to note that these mammals are more closely related to one another than to other mammals. This type of placenta occupies a disc-shaped region of the trophoblast, whence its name discoidal.

At birth, the allantois and placenta are nipped off from the embryo ; and the placenta separates from the uterus, and is expelled as the " after-birth." In the carnivores (type iii) this entails a certain amount of loss of maternal tissue ; in the others the mother only loses blood. In Perameles, on the other hand, the placenta is absorbed by the uterus.

It must be remembered that as well as being an organ of exchange between mother and embryo, the placenta functions during early stages of development as a regulator of meta- bolism of substances such as glycogen. Later on, this function is taken on by the liver of the embryo.

Fig. 119. Section through a part of the allantoic placenta of the cat ; embryonic tissue to the left, maternal to the right.

Letters as Fig. 118.

The Amnion. — The amnion is found only in reptiles, birds, and mammals. All these animals differ from the fish and amphibia in that the eggs are laid on dry land and not in water. The amnion is formed by folds of the extra-embryonic ectoderm and underlying mesoderm which rise up on all sides of the embryo and meet above it. The inner layer so formed en- closing the amniotic cavity is the amnion proper ; the outer layer is the chorion. In the reptiles and the Monotremes, the fusion of the folds above the embryo is not complete, so that the amniotic cavity is not quite closed. In the birds, the amniotic cavity is closed, but it opens again later (at the sero- amniotic connexion).

In the mammals, there are two principal types of amnion- formation. In the one type, of which the rabbit is character- istic, the embryonic plate comes to the surface of the blastocyst by the disappearance of the overlying trophoblast (cells of Rauber), and the amniotic folds rise up on each side of the embryo from the edge of the embryonic plate. This method of formation of the amnion is very similar to that which holds in birds ; the chorion of the latter corresponds to the tropho- blast of the mammals. The only difference is the fact that the trophoblast in the mammal forms a complete investment from the earliest stage.

In the other type, of which the mouse is an example, the amnion arises as a cavity hollowed out in the inner mass of cells, within the trophoblast. This method is called amnion- formation with entypy of the germ. In this case, there are no amniotic folds, and the trophoblast (which forms a complete investment, as in the rabbit) does not become interrupted by any disappearance of Rauber's cells. When the amniotic cavity is formed in the mouse, the embryo becomes differ- entiated on its floor. The mouse therefore starts from a condition which the reptiles do not reach until a fairly late stage of development, when the amniotic folds have been formed.

The amniotic cavity contains fluid, and this enables the embryo to develop in a fluid medium, although its egg was not laid in water.

It is interesting to note how in the higher vertebrates certain processes take place as if they were abbreviations of the conditions which prevail in lower vertebrates. To start with, the primitive streak which is the beginning in higher verte- brates, represents a stage which the lower vertebrates only reach after the blastopore has formed, and become closed by the apposition of its lateral lips. Similarly, the mammal with a hollowed-out amniotic cavity within the trophoblast starts from a condition which the reptiles and birds reach after the upgrowth and fusion of the amniotic folds. Not only this, but in such mammals the amniotic cavity arises first and the embryo forms in its floor ; whereas in the reptiles and birds the embryo forms first and the amniotic folds arise afterwards. This process of " short-circuiting " and telescoping of develop- mental processes reaches its climax in the Primates, where the body-stalk develops first and the allantois grows into it later. In lower mammals as well as in reptiles and birds, the allantois grows out from the hind gut at a fairly late stage. In the Primates, the conditions are as if everything were first got ready for the embryo, after which it makes its appearance. This is not without interest in connexion with the superior organisation and differentiation of the highest mammals, for this superiority in construction is dependent on a prolonged and intense period of embryonic development, when the efficiency of the embryonic membranes and placenta is of the utmost importance.

Literature

Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon Press, 191 3.