We find only in the anthropoid apes—the gibbon and orang of Asia and the chimpanzee and gorilla of Africa—the peculiar and elaborate formation of the placenta that characterises man (Fig. 198). In this case there is at an early stage an intimate blending of the chorion of the embryo and the part of the mucous lining of the womb to which it attaches. The villi of the chorion with the blood-vessels they contain grow so completely into the tissue of the uterus, which is rich in blood, that it becomes impossible to separate them, and they form together a sort of cake. This comes away as the “afterbirth” at parturition; at the same time, the part of the mucous lining of the womb that has united inseparably with the chorion is torn away; hence it is called the _decidua_ (“falling-away membrane”), and also the “sieve-membrane,” because it is perforated like a sieve. We find a decidua of this kind in most of the higher placentals; but it is only in man and the anthropoid apes that it divides into three parts—the outer, inner, and placental decidua. The external or true decidua (Fig. 196 _du,_ Fig. 199 _g_) is the part of the mucous lining of the womb that clothes the inner surface of the uterine cavity wherever it is not connected with the placenta. The placental or spongy decidua (_placentalis_ or _serotina,_ Fig. 196 _ds,_ Fig. 199 _d_) is really the placenta itself, or the maternal part of it (_placenta uterina_)—namely, that part of the mucous lining of the womb which unites intimately with the chorion-villi of the fœtal placenta. The internal or false decidua (_interna_ or _reflexa,_ Fig. 196 _dr,_ Fig. 199 _f_) is that part of the mucous lining of the womb which encloses the remaining surface of the ovum, the smooth chorion (_chorion læve_), in the shape of a special thin membrane. The origin of these three different deciduous membranes, in regard to which quite erroneous views (still retained in their names) formerly prevailed, is now quite clear, The external _ decidua vera_ is the specially modified and subsequently detachable superficial stratum of the original mucous lining of the womb. The placental _decidua serotina_ is that part of the preceding which is completely transformed by the ingrowth of the chorion-villi, and is used for constructing the placenta. The inner _decidua reflexa_ is formed by the rise of a circular fold of the mucous lining (at the border of the _decidua vera_ and _ serotina_), which grows over the fœtus (like the anmnion) to the end.