XXI. ON THE EVOLUTION OF THE PLACENTA, AND ON THE 

 POSSIBILITY OF EMPLOYING THE CHARACTERS OF THE 

 PLACENTA IN THE CLASSIFICATION OF THE MAMMALIA*. 



FROM Owen's observations on the Marsupials it is clear that 

 the yolk-sack in this group plays an important (if not the most 

 important) part, in absorbing the maternal nutriment destined 

 for the foetus. The fact that in Marsupials both the yolk-sack 

 and the allantois are concerned in rendering the chorion vascular, 

 makes it a priori probable that this was also the case in the 

 primitive types of the Placentalia ; and this deduction is sup- 

 ported by the fact that in the Rodentia, Insectivora, and Cheiro- 

 ptera this peculiarity of the foetal membranes is actually found. 

 In the primitive Placentalia it is also probable that from the 

 discoidal allantoic region of the chorion simple foetal villi, like 

 those of the Pig, projected into uterine crypts ; but it is not 

 certain how far the umbilical region of the chorion, which was 

 no doubt vascular, may also have been villous. From such a 

 primitive type of fcetal membranes divergencies in various 

 directions have given rise to the types of foetal membranes found 

 at the present day. 



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 foetal villi and maternal crypts over a limited 

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

 by the placental villi. Various combinations of the two pro- 

 cesses would also, of course, be advantageous. 



1 From the Proceedings of the Zoological Society of London, t88i. 



