236 



EMBRYOLOGY. 



In the Mammalia with a decidua we must distinguish two sub- 

 types of placenta, a ring-like and a disc-like, a placenta zonaria and 

 a placenta discoidea. 



The placenta zonaria is characteristic of the Carnivora. The 

 blastodermic vesicle in this case generally has the shape of a cask. 

 With the exception of both poles, which retain a smooth surface, the 

 chorion is covered with numerous villi arranged in a girdle -shaped 

 zone ; the villi are furnished with lateral branches, like a tree. 



The branched villi of the chorion sink into the thickened mucous 



Fig. 136. Diagrammatic representation of the finer structure of the placenta of a Cow, after 



TURNER. 

 F, Foetal, M, maternal placenta ; V, villus ; e, epithelium of the chorionic villus ; e', epithelium 



of the maternal placenta ; d, foetal, d', maternal blood-vessels. 



Fig. 137. Diagrammatic representation of the finer structure of the placenta of a Cat, after 

 TURNER. Explanation of letters as in fig. 13C. 



membrane of the uterus in various directions, so that in sections 

 there arises the appearance of an irregular interlacing (fig. 137). 

 However, according to the concurrent accounts of TURNER and 

 ERCOLANI, there is no penetration into the uterine glands in this case, 

 any more than in the case of the Indeciduata. 



The epithelium (e') of the maternal mucous membrane (M ) persists 

 and forms a boundary between the villi ( V) and the maternal blood- 

 vessels (d'), which latter have enlarged to cavities from three to four 

 mites as wide as the foetal capillaries (d). This enlargement of the 



