Development of the Mammal 301 



important part in the formation of the foetal 

 placenta, that part of the organ of attachment 

 that is derived from the embryo, in distinction 

 to the maternal placenta which is a specially 

 modified region of the uterus. A careful study 

 of Figures 93-96 will make plain the similarity 

 in the formation of the false amnion or chorion 

 in the mammal and in the chick. 



In the chick, as has been noted above, the 

 chorion remains as a thin, smooth membrane, 

 but in the mammal its outer surface soon be- 

 comes roughened by small thickenings, which 

 thickenings become large and branched to form 

 the chorionic villi. That part of the chorion 

 which lies next to the uterine wall, and is most 

 intimately associated with the formation of the 

 placenta, retains the villi as large, vascular, 

 branching turfs and is known as the chorion fron- 

 dosum. That part of the chorionic vesicle that 

 is away from the region of attachment to the 

 wall of the uterus is the chorion Iceve (Fig. 97). 



It will be well, at this point, to say something 

 of the position of the embryo in the uterus, 

 and of the uterine walls during pregnancy. 

 The " implantation " of the ovum by the action 

 of the trophoblast has already been mentioned. 

 Jn some animals the ovum, even after complete 



