216 BIOLOGY: GENERAL AND MEDICAL 



which apparently melts away below its attachment in 

 order that placentation may be made possible later on. 



A free ovum in water or an ovum inclosed within a 

 shell is able to perfect its differentiations undisturbed 

 by contact with external bodies; but a mammalian ovum 

 of small size, without a nutrient yolk to feed upon and 

 situated in a crypt of the uterine mucosa, is obliged to 

 prepare for its future nutrition by effecting a communi- 

 cation with the maternal supply, and arrange for its 

 freedom from external interference by surrounding 

 itself with smooth membranes within which develop- 

 ment may proceed. 



Fish embryos are without such membranes; the 



Vacuolesin ettibrrotrtC fxjffbit 



Fio. 94. Ovum of bat; differentiation of amniotic cavity. X 275. (Van 



Beneden.) 



chick forms one of them, the amnion; the mammalian 

 ovum forms two, the amnion for protection and the 

 chorion for nutrition. It is not necessary to particu- 

 larize in regard to the amnion or other foetal membranes 

 as this chapter is not intended to be an adequate descrip- 

 tion of the developmental details, but simply to epitomize 

 such facts appertaining to development as shall show 

 the harmony existing between all forms of life in the 

 general plan upon which ontogeny is perfected. 



With the formation of the amnion, the embryo ad- 

 vances to the point where it consists of a didermic plate, 

 on the uterine side of which the primitive amniotic 

 cavity is situated, on the other side of which is the yolk 

 sac found. 



