In the Marsupialia the observations made many years ago by Sir R. Owen upon the development of the Kangaroo have been confirmed by those of Dr. H. C. Chapman,[18] while Dr. Selenka,[19] and Professor H. F. Osborn[20] have contributed important evidence as to the structure and relations of the fœtal membranes of the Opossums and others. It thus appears that up to the period of the very premature birth of these animals the outer covering of the ovum, or false chorion, is free from persistent villi, and not adherent to the epithelium of the uterine walls; for, although fitting into the folds of the latter, it is perfectly and readily separable in its entire extent from them. The umbilical vesicle or yolk-sac is large, vascular, and adherent to a considerable portion of the false chorion or subzonal membrane, while the allantois is relatively small, and although the usual blood-vessels can be traced into it, it does not appear to contract any connection with the false chorion, and, therefore, much less with the walls of the uterus, of such a nature as to constitute a placenta. In some forms, however, such as the Opossums, the umbilical vesicle or yolk-sac develops temporary villi, which unite with the subzonal membrane, or false chorion, to form a disc-like area closely attached to the cells covering the utricular glands of the uterine epithelium, and thus forming a so-called _yolk-sac placenta_. The function of this organ is considered to be the transmission of the secretions of the utricular glands to the embryo by means of the umbilical vesicle; the function of the allantois being either respiratory or the absorption of the fluid secreted in the uterine cavity by the utricular glands. While in the uterus the nourishment of the fœtus 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 respiration and the lacteal secretion of the mother very early superseding 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.