ORGANS.] MAMMALIA The nourishment of the foetus during intra-uterine life takes place through the medium of certain structures, partly belonging to the foetus itself and partly belonging to the inner parietes of the uterus of the parent. These in their complete form constitute the complex organ called the "placenta," which serves as the medium of communication between the mother and foetus, and in which the physio logical processes that are concerned in the nutrition of the latter take place ; but, as we shall see, though a placenta, in the usual acceptation of the term, is peculiar to the mammalian class, it is not in all of its members that one is developed. The structures to which we shall have especially to refer are the outer tunic of the ovum, to which, however formed, the term " chorion " is commonly applied, and two sac-like organs connected with the body cavity of the em bryo, both formed from the splanchnic mesoblast, lined by a layer of the hypoblast. These are the " umbilical vesicle " or " yolk sac " and the " allantois." The umbilical vesicle is a thin membrane enclosing the yolk, which by the doubling in of the ventral walls of the embryo becomes gradually formed into a distinct sac external to the body, with a pedicle (the omphalo-enteric duct) by which for a time a communication is maintained between its cavity and the intestinal canal. In the walls of this sac blood-vessels (omphalo-meseraic or vitelline) are developed in connexion with the vascular system of the embryo, through which, either by their contact with the outer surface of the walls of the ovum, or by the absorption through them of the contents of the yolk sac, the nutrition of the embryo in the lower vertebrates chiefly takes place. In mammals the umbilical vesicle plays a comparatively subordinate part in the nourishment of the foetus, its function being generally superseded by the allantois. The last-named sac commences at a very early period as a diverticulum from the hinder end of the alimentary tract of the embryo. Its proximal portion afterwards becomes the urinary bladder, the contracted part between this and the cavity of the allantois proper constituting the urachus, which passes out of the body of the foetus at the umbilicus together with the vitelline duct. The mesoblastic tissue of the walls of the allantois soon becomes vascular; its arteries are supplied with foetal blood by the two hypogastric branches of the iliacs, or main divisions of the abdominal aorta, and the blood is returned by venous trunks which unite to form the single umbilical vein which runs to the under surface of the liver, where, part of it joining the portal vein and part entering the vena cava directly, it is brought to the heart. These are the vessels which, with their surrounding membranes, constitute the umbilical cord, the medium of communication between the foetus and the placenta, when that organ is fully de veloped. The nature of the foetal membranes of the Monotremata is not known. In the Marsupialia the observations made many years ago by Professor Owen, upon the development of the Kangaroo, have recently been confirmed by Dr H. C. Chapman, 1 but fuller investigations in different species and at different stages are still much to be desired. As far as is known, up to the period of the very premature birth of these animals the outer covering of the ovum or chorion is free from villi and not adherent to the uterine walls, for, though fitting into the folds of the latter, it is perfectly and readily separable in its entire extent from them. The umbilical vesicle is large, vascular, and adherent to a con siderable portion of the chorion, while the allantois is relatively small, and, though the usual blood-vessels can 1 Proc. Acad. Nat. Sc. Philadelphia, December 27, 1881 ; Ann. Mag. Nat. Hist., April 1882. be traced in it, it does not appear to contract any con nexion with the chorioo, and therefore much less with the walls of the uterus, of such a nature as to constitute a placenta. While in the uterus the nourishment of the foetus seems therefore to be derived from the umbilical vesicle, as in reptiles and birds, rather than from the uterine walls by means of the allantoic vessels, as in the higher mammals. The latter vessels, in fact, play even a much less important part in the development of these animals, not only than in the placental mammals, but even than in the Sauropsida, for they can scarcely have the respiratory function assigned to them in that group. Pulmonary re spiration and the lacteal secretion of the mother very early supersede all other methods of providing the due supply both of oxygen and of food required for the development and growth of the young animal. In this sense the Marsupials may be looked upon as the most typically " mammalian " of the whole class. In no other group do the milk-secreting glands play such an important part in providing for the continuity of the race. In the thi>*d primary division of the Mammalia, the so- called Placentalia, the umbilical vesicle generally does not quite unite with the chorion, and disappears as develop ment proceeds, so that no trace of it can be seen in the membranes of an advanced embryo ; but it may persist until the end of intra-uterine life as a distinct sac in the umbilical cord, or lying between the allantois and amnion. The disappearance or persistence of the umbilical vesicle does not. according to our present knowledge, appear to be correlated with a higher or lower general grade of development, as might be presupposed. It is stated to have been found in Man even up to the end of intra-uterine life, and also in the Carnivora, while in the Ungulata and Cetacea it disappears at an earlier age. In many, if not all, of the Rodentia, Insectivora, and Chiroptera, it plays a more important part, becoming adherent to a considerable part of the inner surface of the chorion, to which it conveys blood-vessels, although villi do not appear to be developed from the surface of this part, as they are on the portion of the chorion supplied by the allantoic vessels. These orders thus present to a certain extent a transitional condition from the Marsupials, although essentially different, in possessing the structures next to be described. The special characteristic of the whole of the placental mammals constituting the majority of the class, is that the allantois and its vessels become intimately blended with a smaller or greater part of the parieties of the ovum, form ing a structure on the outer surface of which villi are de veloped, and which, penetrating into corresponding cavities of the " decidua," or soft, vascular, hypertrophied lining membrane of the uterus, constitutes the placenta. This organ may be regarded, as Professor Turner says, both in its function and in the relative arrangement of its consti tuent textures, as a specially modified secreting gland, the ducts of which are represented by the extremities of the blood-vessels of the foetal system. The passage of material from the maternal to the foetal system of vessels is not a simple percolation or diffusion through their walls, but is occasioned by the action of a layer of cells derived from the maternal or uterine structures, and interposed between the blood-vessels of the maternal part of the placenta and those of the villi covering the chorion, in which the embryonic vessels ramify. The numerous modifications in the details of the structure of this organ relate to increasing the absorbing capacity of the vessels of the chorion, and are brought about either by increasing the complexity of the fcetal villi and maternal crypts over a limited area, or by increasing the area of the part of the chorion covered by the placental villi, or by various combinations of the two methods.
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