84 



COMPARATIVE PHYSIOLOaY. 



membranes. It will be observed from the figure tbat the true 

 villous chorion is confined to a comparatively small region; 

 there is, however, in addition a false chorion without villi, but 

 highly vascular. This blending of forms of placentation which 

 exist separately in different groups of animals is significant. 

 In the rabbit at a later stage there is considerable interming- 

 ling of fcetal and maternal parts. 



^^^■^%—Sepes of diagrams representing the relations of the decidua to the ovum, at 

 different periods, in the human subject. The decidua are dark, the ovum shaded 

 transversely. In 4 and 5 the chorionic vascular processes are figured (after Dal- 

 ton;. 1. Ovum resting on the decidua serotina; 2. Decidua reflexa growing round 

 tne ovum; 3. Complefion of the decidua around the ovum: 4. Villi, growing out 

 all around the chorion; 5. The villi, specially developed at the site of the future 

 placenta, having atrophied elsewhere. 



The Metadiscoidal Placenta.— This type, which, in general 

 naked-eye appearances, greatly resembles the former, is found 

 in man and the apes. The condition of things in man is by no 

 means as well understood as in the lower mammals, especially 

 in the early stages ; so that, while the following account is that 



