36-4 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. XI. 



Evolution of the placenta. Excluding the mar- 

 supials whose placentation is not really known, the 

 arrangement of the foetal membranes of the Rabbit is 

 the most primitive observed. In this type the allantois 

 and yolk-sac both function in obtaining nutriment 

 from the mother ; and the former occupies only a small 

 discoidal area of the subzonal membrane. In all higher 

 types the allantois gradually spreads out over the whole 

 inner surface of the subzonal membrane and its im- 

 portance increases ; while that of the yolk-sac as a nu- 

 tritive organ decreases. In the diffuse type of placenta 

 simple villi are present over nearly the whole surface of 

 the chorion. In the remaining types the vUli become 

 more complicated and restricted to a smaller area 

 (meta-discoidal, zonary, &c.) of the chorion ; though in 

 the early stages they are more scattered and simpler, 

 in some cases occupying nearly the whole surface of the 

 chorion. It therefore seems probable that the placenta 

 of Man has been derived not directly from the discoidal 

 placenta of the Rabbit, but from the diffuse placenta 

 such as is seen in the Lemurs, etc., and that generally 

 the zonary, cotyledonary, &c. types of placenta have 

 been derived from the diffuse by a concentration and 

 increase in the complexity of the foetal villi. 



