358 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 



cidua vera, and reflexa, with which it is continuous, is shed ; and 

 the blood-vessels thus ruptured are closed by the contraction of 

 the uterine walls. 



The metadiscoidal placenta of Man and Apes and the discoidal 

 placenta of the Eabbit are usually classified by anatomists as 

 discoidal placentae, but it must be borne in mind that they differ 

 very widely. 



In the Eabbit there is a dorsal placenta, which is co- extensive 

 with the area of contact between the allantois and the subzonal 

 membrane, while the yolk-sac adheres to a large part of the 

 subzonal membrane. In Apes and Man the allantois spreads 

 over the whole inner surface of the subzonal membrane ; the 

 placenta is on the ventral side of the embryo, and occupies only a 

 small part of the surface of the allantois. 



Zonary placenta. Another form of deciduate pla- 

 centa is known as the zonary. This form of placenta 

 occupies a broad zone of the chorion, leaving the two 

 poles free. It is found in the Carnivora, Hyrax, Elephas, 

 and Orycteropus. 



In the Dog, which may be taken as a type, there is a large 

 vascular yolk-sac formed in the usual way, which does not how- 

 ever fuse with the chorion. It has at first an oval shape, and 

 persists till birth. The allantois first grows out on the dorsal 

 side of the embryo, where it coalesces with the subzonal mem- 

 brane, over a small discoidal area, and there is thus formed a 

 rudimentary discoidal placenta closely resembling that of the 

 Rabbit. 



The area of adhesion between the outer part of the allantois 

 and subzonal membrane gradually spreads over the whole inte- 

 rior of the subzonal membrane, and vascular villi are formed over 

 the whole area of adhesion except at the two extreme poles of the 

 ovum. 



With the full growth of the allantois there is formed a broad 

 placental zone, with numerous branched villi fitting into corre- 

 sponding pita which are not true glands but special develop- 



