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 foetal 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 in- 

 creasing 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 dimi- 

 nution 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 Lemuridse, the Suidse, and Manis. 

 In all these cases the area of the placental villi appears to have in- 

 creased 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, differ- 

 entiations 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 foetal 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 Man 

 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 con- 

 centrations 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 

 Mollis. 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 mdacaies 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 placentEe of the Armadillos, Myrmecophaga, and 

 the Sloths may easily have been formed from a diffused placenta, 

 just as the discoidal placenta of the Simiidse and HominidBe 

 appears to have been formed from a diffused placenta like that 

 of the Lemuridse. 



The presence of zonary placentae in Ilyrax and Elephas does not 

 necessarily afford any proof of affinity of these types with the Carni- 

 vora. A zonary placenta may be quite as easily derived from a 

 diffused placenta as from a discoidal placenta j 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 dif- 

 fused placenta. 



Although it would not be wise to attempt to found a classification 

 upon the jdacental characters alone, it may be worth while to make 



14* 



