“The deciduate placenta is a distinct structure, developed from the wall of the uterus, but separated from it at parturition, and constituting what is known as the ‘after birth’; of this the human placenta is regarded by Huxley as the most perfect example; while, of the non-deciduate placenta, that of the pig and horse are the typical representatives. The word ‘decidua’ signifies ‘that which is thrown off.’ “_The Non-Deciduate Placenta._—This form is thus described by Professor Huxley: ‘No decidua is developed. The elevations and depressions of the unimpregnated uterus simply acquire a greater size and vascularity during pregnancy, and cohere closely to the chorionic villi, which do not become restricted to one spot, but are developed from all parts of the chorion, except at its poles, and remain persistent in the broad zone thus formed throughout fœtal life. The cohesion of the fœtal and maternal placentæ, however, is overcome by slight maceration; and at parturition the fœtal villi are simply drawn out, like fingers from a glove, no vascular substance of the mother being thrown off.’ To this class belong all the ruminants and Ungulata (hoofed quadrupeds); the camel, sheep, goat and deer; the ant-eater, armadillo, sloth, swine, tapir, rhinoceros, river-horse, sea-cow, whale, and others. “_The Zonary Deciduate Placenta._—A zonary placenta surrounds the chorion, in the form of a broad zone, leaving the poles free. This form characterises all the land and sea carnivora, and thus includes the cat, hyena, puma, leopard, tiger, lion, fox and wolf; the dog and bear, the seal, sea-otter and walrus. It includes, also, certain extinct species, as the mastadon and dinotherium, which, although not wholly carnivorous, were, to judge from their teeth, partially so. The elephant, the only living species of these ancient animals, is also of this class. “_The Discoidal Deciduate Placenta._—The discoidal placenta is a highly developed vascular structure, lying on one side of the fœtus, in the form of a round disc, leaving the greater part of the chorion free. It is thus united only on one side, at one circular point, with the mucous membrane of the uterus, from which, as already mentioned, it is separated at parturition. The orders of the animals characterised by this form of placenta are the rodentia, ant-eaters, bats, and various species of apes, and man. All these are very closely united by homologous anatomical forms. The human placenta does not differ, in its general character, from that of the others, and there is no good reason for separating man from his placental classification.”