210 MR. F. M. BALFOUR ON THE [Feb. t, 



February 1, 1881. 

 Professor Flower, LL.D., F.R.S., President, in the Chair. 



The following papers were read : — 



1. On the Evolution of the Placenta, and on the possibility 

 of employing the characters of the Placenta in the 

 Classification of the Mammalia. By F. M. Balfour, 

 M.A., F.R.S., Fellow of Trinity College, Cambridge. 



[Eeceived January 22, 1881.] 



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

 yolk-sac 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-sac and the allantois are 

 concerned in rendering the chorion vascular, makes it a priori pro- 

 bable that this was also the case in the primitive types of the 

 Placentalia ; and this deduction is supported by the fact that in the 

 Rodentia, Insectivora, and Cheiroptera 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 i)rimitive 

 type of foetal membraues divergences 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. (1) by increasing the complexity of 

 the foetal villi and maternal crypts over a limited area, (2) by .in- 

 creasing the area of the part of the chorion covered by the 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 foetal parts in the Rodentia, Insectivora, 

 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-sac 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 pla- 

 centa, as evinced by the fact that in the growth of the placenta the 



