Development of the Mammal 311 



vessels and sinuses have developed ; while the 

 latter is the thickened, discoidal portion of 

 the chorion at the end of the umbilical cord. 

 The size of the foetal placenta varies consider- 

 ably, but is usually about 17 cm. in diameter 

 and 25 mm. thick. It is oval or circular in 

 outline, and the cord is generally attached 

 eccentrically. The side away from the wall 

 of the uterus and towards the embryo is more 

 or less smooth, except where the ramifying 

 vessels from the cord spread over it as small 

 ridges (Fig. 101) ; this side is covered by the 



FIG. 102. HUMAN EMBRYO OF 2.6 MM. (From Minot, after His.) 



