THE EVOLUTION OF THE PLACENTA. 735 



The most fundamental change which has taken place in all 

 the existing Placentalia is the exclusion of the umbilical vesicle 

 from any important function in the nutrition of the fcetus. 



The arrangement of the fcetal parts in the Rodentia, In- 

 sectivora, and Cheiroptera may be directly derived from the 

 primitive form by supposing the villi of the discoidal placental 

 area to have become more complex, so as to form a deciduate 

 discoidal placenta, while the yolk-sack still plays a part, though 

 physiologically an unimportant part, in rendering the chorion 

 vascular. 



In the Carnivora, again, we have to start from the discoidal 

 placenta, as evinced by the fact that in the growth of the pla- 

 centa the 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 nutri- 

 tion of the fcetus by insuring a more intimate relation between 

 the fcetal and maternal parts, than by increasing their area of 

 contact. The reason of this is not obvious, but, as shewn 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 Man has no connexion with, and 

 ought not to be placed in, the same class as those of the Ro- 

 dentia, Cheiroptera, and Insectivora. 



