236 



EMBRYOLOGY. 



In the Mammalia with a decidua we must distinguish tivo 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. 137. 



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



TURNER. . 

 P, Foetal, M, maternal placenta ; V, villus ; e, epithelium of the chorionic villua ; 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. 136. 



membrane of the uterus in various directions, sojthat_in sections 

 there arises the appearance of ala Irregular interlacing (fig. T37). 

 However, according to the concurrent accounts of TURNER and 

 y 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 1 ), which latter have enlarged to cavities from, three to four 

 mites as wide as the fo2tal capillaries (d). This enlargement of the 



