1. The non-deciduate placenta of the Herbivora.
2. The deciduate placenta, of which there are two kinds:
(a) The zonary deciduate placenta of the Carnivora.
(b) The discoidal deciduate placenta of the Frugivora.
“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.”
Relations between placental forms and Individual Characteristics.—From our entire knowledge of the development of races and of individuals, we may conclude, upon the basis of Huxley’s classification, that an intimate relation exists between the form and character of the placenta and the entire nature of the individual. We find among the non-deciduata, besides the toothless sloths, only the Ungulata, or hoofed quadrupeds, and others developed from them. The arrangement of their teeth, as of their entire digestive apparatus, marks them as belonging to a single family—namely, the herbivora.
The zonary placenta characterises a very large family of animals whose peculiarities are distinctly marked, especially with regard to their teeth and digestive apparatus. These belong to the widely diffused and numerous orders of the carnivora. But the most interesting and important group, with reference to our present study, is that characterised by the discoidal placenta; for, since it includes man and the fruit-eating apes, it gives occasion for the comparison between these and other placental animals from the standpoint of dietetics.