XI.J NON-DECIDUATE PLACENTA. 359 



ments of the uterine surface. The maternal and foetal structures 

 become closely interlocked and highly vascular ; and at birth a 

 large part of the maternal part is carried away with the placenta ; 

 some of it however stiU remains attached to the muscular wall of 

 the uterus. The zone of the placenta diminishes greatly in pro- 

 portion to the chorion as the latter elongates, and at the fuU 

 time the breadth of the zone is not more than about one-fifth of 

 the whole length of the chorion. 



At the edge of the placental zone there is a very small portion 

 of the uterine mucous membrane reflected over the non-placental 

 part of the chorion, so as to form a small reflexa analogous with 

 the reflexa in Man. 



The most important of the remaining types of pla- 

 centa are the difflise and the polycotyledonary, and 

 these placentse are for the most part non-deciduate. In 

 the diffuse placenta, found in the Horse, Pig, Le- 

 murs, etc., the allantois completely envelopes the em- 

 bryo, and villi are formed on all parts of the chorion, 

 excepting over a small area at the two poles. 



In the polycotyledonary placenta, vphich is charac- 

 teristic of the Ruminantia, the allantois grows round the 

 whole inner surface of the subzonal membrane ; the 

 placental villi are however not uniformly distributed, 

 but collected into patches or cotyledons, which form as 

 it were so many small placentse. The fcetal villi of 

 these patches fit into corresponding pits in thickened 

 patches of the wall of the uterus. 



Comparative histology of the Placenta. 



It does not fall within the province of this work to 

 treat from a histological standpoint the changes which 

 take place in the uterine walls during pregnancy. It 

 will, however, be convenient to place before the reader 



