_Placenta._—The development of the mammalian ovum, and the changes which the various tissues and organs of the body undergo in the process of growth, are too intricate subjects to be explained without entering into details incompatible with the limits of this work, especially as they scarcely differ, excepting in their later stages, from those of other vertebrates, upon which, owing to the greater facilities these present for examination and study, the subject has been more fully worked out. There are, however, some points which require notice, as peculiar to the mammalian class, and as affording at least some hints upon the difficult subject of the affinities and classification of the members of the group. The nourishment of the fœtus during intra-uterine life takes place through the medium of certain structures, partly belonging to the fœtus 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,” serving as the medium of communication between the mother and fœtus, and in which the physiological 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 embryo, 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 connection 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 fœtus, its function being generally superseded by the allantois.