While the above changes are taking place in the amnion, the allantois grows out from the hind gut as a vesicle lined by hypoblast, but covered externally by a layer of splanchnic mesoblast ([fig. 147], 3 and 4, al)[83]. The allantois soon becomes a flat sack, projecting into the now largely developed space between the subzonal membrane and the amnion, on the dorsal side of the embryo ([fig. 147*], ALC). In some cases it extends so as to cover the whole inner surface of the subzonal membrane; in other cases again its extension is much more limited. Its lumen may be retained or may become nearly or wholly aborted. A fusion takes place between the subzonal membrane and the adjoining mesoblastic wall of the allantois, and the two together give rise to a secondary membrane round the ovum, known as the chorion. Since however the allantois does not always come in contact with the whole inner surface of the subzonal membrane, the term chorion is apt to be somewhat vague; and in the rabbit, for instance, a considerable part of the so-called chorion is formed by a fusion of the wall of the yolk-sack with the subzonal membrane ([fig. 148]). The placental region of the chorion may in such cases be distinguished as the true chorion, from the remaining part which will be called the false chorion.
The mesoblast of the allantois, especially that part of it which assists in forming the chorion, becomes highly vascular; the blood being brought to it by two allantoic arteries continued from the terminal bifurcation of the dorsal aorta, and returned to the body by one, or rarely two, allantoic veins, which join the vitelline veins from the yolk-sack. From the outer surface of the true chorion ([fig. 147], 5, d, [148]) villi grow out and fit into crypts or depressions which have in the meantime made their appearance in the walls of the uterus[84]. The villi of the chorion are covered by an epithelium derived from the subzonal membrane, and are provided with a connective tissue core containing an artery and vein and a capillary plexus connecting them. In most cases they assume a more or less arborescent form, and have a distribution on the surface of the chorion varying characteristically in different species. The walls of the crypts into which the villi are fitted also become highly vascular, and a nutritive fluid passes from the maternal vessels of the placenta to the fœtal vessels by a process of diffusion; while there is probably also a secretion by the epithelial lining of the walls of the crypts, which becomes absorbed by the vessels of the fœtal villi. The above maternal and fœtal structures constitute together the organ known as the placenta. The maternal portion consists essentially of the vascular crypts in the uterine walls, and the fœtal portion of more or less arborescent villi of the true chorion fitting into these crypts.
While the placenta is being developed, the folding-off of the embryo from the yolk-sack becomes more complete; and the yolk-sack remains connected with the ileal region of the intestine by a narrow stalk, the vitelline duct ([fig. 147], 4 and 5 and [fig. 147*]), consisting of the same tissues as the yolk-sack, viz. hypoblast and splanchnic mesoblast. While the true splanchnic stalk of the yolk-sack is becoming narrow, a somatic stalk connecting the amnion with the walls of the embryo is also formed, and closely envelops the stalk both of the allantois and the yolk-sack. The somatic stalk together with its contents is known as the umbilical cord. The mesoblast of the somatopleuric layer of the cord develops into a kind of gelatinous tissue, which cements together the whole of the contents. The allantoic arteries in the cord wind in a spiral manner round the allantoic vein. The yolk-sack in many cases atrophies completely before the close of intra-uterine life, but in other cases it is only removed with the other embryonic membranes at birth. The intra-embryonic portion of the allantoic stalk gives rise to two structures, viz. to (1) the urinary bladder formed by a dilatation of its proximal extremity, and to (2) a cord known as the urachus connecting the bladder with the wall of the body at the umbilicus. The urachus, in cases where the cavity of the allantois persists till birth, remains as an open passage connecting the intra- and extra-embryonic parts of the allantois. In other cases it gradually closes, and becomes nearly solid before birth, though a delicate but interrupted lumen would appear to persist in it. It eventually gives rise to the ligamentum vesicæ medium.
At birth the fœtal membranes, including the fœtal portion of the placenta, are shed; but in many forms the interlocking of the fœtal villi with the uterine crypts is so close that the uterine mucous membrane is carried away with the fœtal part of the placenta. It thus comes about that in some placentæ the maternal and fœtal parts simply separate from each other at birth, and in others the two remain intimately locked together, and both are shed together as the afterbirth. These two forms of placenta are distinguished as non-deciduate and deciduate, but it has been shewn by Ercolani and Turner that no sharp line can be drawn between the two types; moreover, a larger part of the uterine mucous membrane than that forming the maternal part of the placenta is often shed in the deciduate Mammalia, and in the non-deciduate Mammalia it is probable that the mucous membrane (not including vascular parts) of the maternal placenta either peels or is absorbed.
Comparative history of the Mammalian fœtal membranes.
Two groups of Mammalia—the Monotremata and the Marsupialia—are believed not to be provided with a true placenta.
The nature of the fœtal membranes in the Monotremata is not known. Ova, presumably in an early stage of development, have been found free in the uterus of Ornithorhyncus by Owen. The lining membrane of the uterus was thickened and highly vascular. The females in which these were found were killed early in October[85].
Marsupialia. Our knowledge of the fœtal membranes of the Marsupialia is almost entirely due to Owen. In Macropus major he found that birth took place thirty-eight days after impregnation. A fœtus at the twentieth day of gestation measured eight lines from the mouth to the root of the tail. The fœtus was enveloped in a large subzonal membrane, with folds fitting into uterine furrows, but not adhering to the uterus, and without villi. The embryo was enveloped in an amnion reflected over the stalk of the yolk-sack, which was attached by a filamentary pedicle to near the end of the ileum. The yolk-sack was large and vascular, and was connected with the fœtal vascular system by a vitelline artery and two veins. The yolk-sack was partially adherent, especially at one part, to the subzonal membrane. No allantois was observed. In a somewhat older fœtus of ten lines in length there was a small allantois supplied by two allantoic arteries and one vein. The allantois was quite free and not attached to the subzonal membrane. The yolk-sack was more closely attached to the subzonal membrane than in the younger embryo[86].
All Mammalia, other than the Monotremata and Marsupialia, have a true allantoic placenta. The placenta presents a great variety of forms, and it will perhaps be most convenient first to treat these varieties in succession, and then to give a general exposition of their mutual affinities[87].
Amongst the existing Mammals provided with a true placenta, the most primitive type is probably retained by those forms in which the placental part of the chorion is confined to a comparatively restricted area on the dorsal side of the embryo; while the false chorion is formed by the vascular yolk-sack fusing with the remainder of the subzonal membrane. In all the existing forms with this arrangement of fœtal membranes, the placenta is deciduate. This, however, was probably not the case in more primitive forms from which these are descended[88]. The placenta would appear from Ercolani’s description to be simpler in the mole (Talpa) than in other species. The Insectivora, Cheiroptera, and Rodentia are the groups with this type of placenta; and since the rabbit, amongst the latter, has been more fully worked out than other species, we may take it first.