260 EVOLUTION OF THE PLACENTA. 



villous. From such a primitive type of foetal membranes 

 divergences in various directions have given rise to the types of 

 foetal membranes now existing. 



In a general way it may be laid down that variations in any 

 direction which tended to increase the absorbing capacities of 

 the chorion would be advantageous. There are two obvious 

 ways in which this might be done, viz. (i) by increasing the 

 complexity of the fcetal villi and maternal crypts over a limited 

 area, (2) by increasing the area of the part of the chorion 

 covered by placental villi. Various combinations of the two 

 processes would also of course be advantageous. 



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 foetus. 



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 shewn by the fact that the allantoic region of the 

 placenta is at first discoidal (p. 248). 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 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 

 mentioned below, there are other cases where it can be shewn 

 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 

 discoidal 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 



