ONTOGENESIS 



229 



freedom from external interference by surrounding itself 

 with smooth membranes within which development may 

 proceed. 



Fish embryos are without such membranes; the 



FIG. 95. Section through embryonic region of ovum. First week of preg- 

 nancy. E.Sch, Embryonic epiblast; Ent, embryonic hypoblast; Mes, embryonic 

 mesoblast; D.S, umbilical vesicle; Ekt, chorionic epiblast; Sp, fold in exoccelom; 

 A.H, amniotic cavity lined by a single layer of flattened cells, which are in strik- 

 ing contrast with the layer of cylindric cells of the embryonic epiblast. (H. 

 Peters.) 



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. 



