INTERRELATIONS OF THE MESONEPHROS, ETC. 195 



It will be seen at once that in the animals selected for study 

 there is a close relationship between the size and duration of 

 the Wolffian body and the size of the allantois. The cat has a 

 smaller allantois than the pig or sheep, because its Wolffian 

 body is less effective, but a larger one than the rabbit since the 

 urine is accumulated throughout intra-uterine life in the cat, 

 but only for a short period in the rabbit. 



THE PLACENTA 



The placentae of various mammals have been studied anatomi- 

 cally and physiologically by many of the best investigators, and 

 the different types of placenta belonging to the different groups 

 of mammals are well known. Grosser, one of the most recent 

 writers on the subject, arranges the types as follows: 



A. Semiplacentae (placentae appositae) 



a) semiplacenta diffusa. Type; pig. 



b) semiplacenta multiplex. The ruminants. 



B. Placentae verae (conjugatae) 



a) placenta zonaria. Type; most carnivora, cat. 



b) placenta discoidalis. 



1. Rodents, rabbit, mouse, guinea pig. 



2. Insectivora. 



3. Cheiroptera. 



4. Primates. 



In the apposed placenta, as its name implies, there is no definite 

 union between the fetal and maternal tissues, no destruction 

 of the maternal epithelium by the trophoderm of the chorion. 

 The chorionic epithelium is apposed to the epithelium of the 

 uterus, perhaps sending cell processes between the maternal 

 cells, as maintained by Robinson, but separating from the ma- 

 ternal cells at the end of pregnancy without destruction of the 

 uterine surface. There is no intimate relation between the 

 maternal blood and the fetal vessels; nutrition and oxygenation 

 of the embryo go on by means of the active absorption by the 

 chorionic epithelial cells of products directly given to them 

 by the maternal epithelium, or of the ^iterine milk,' the excre- 



