OCR Text |
Show 1881.] EVOLUTION OF THE PLACENTA. 211 allantoic region of the placenta is at first discoidal, and only becomes zonary at a later stage. A zonary deciduate placenta indicates an increase both in area and in complexity. The relative diminution of the breadth of the placental zone in late fcetal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the foetus by ensuring a more intimate relation between the foetal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but, as shown below, there are other cases where it is clear that a diminution in the area of the placenta has taken place, accompanied by an increase in the complexity of its villi. The second type of differentiation from the primitive form of placenta is illustrated by the Lemuridae, the Suidae, and Manis. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great extent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. The placenta of Man and Apes, from its mode of ontogeny, is clearly derived from a diffused placenta (very probably similar to that of Lemurs) by a concentration of the fcetal villi, which are originally spread over the whole chorion, to a disk-shaped area, and by an increase in their arborescence. Thus the discoidal placenta of M an has no connexion with, and ought not to be placed in, the same class as those of the Rodentia, Cheiroptera, and Insectivora. The polycotyledonary forms of placenta are due to similar concentrations of the foetal villi of an originally diffused placenta. In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta such as that found in Manis. The zonary placenta of Orycteropus is capable of being easily derived from that of Manis by the disappearance of the foetal villi at the two poles of the ovum. The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that of the Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentas of the Armadillos, Myrmecophaga, and the Sloths may easily have been formed from a diffused placenta, just as the discoidal placenta of the Simiidae and Hominidae appears to have been formed from a diffused placenta like that of the Lemuridae. The presence of zonary placentae in Hyrax and Elephas does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may be quite as easily derived from a diffused placenta as from a discoidal placenta; and the presence of two villous patches at the poles of the chorion in Elephas very probably indicates that its placenta has been evolved from a diffused placenta. Although it would not be wise to attempt to found a classification upon the placental characters alone, it may be worth while to make 14* |