ONTOGENESIS 



217 



This didermic plate, consisting of ectoderm and ento- 

 derm, between which the cells of the mesoderm soon 

 make their appearance, is not essentially different from 

 the oval germinal disc floating upon the great yolk of the 

 hen's egg, and its future development progresses in much 

 the same way. In the mammalian egg, however, the 

 researches of Peters and van Beneden indicate that 

 endoderm formation does not take place through gas- 



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 exocoelom; 

 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. (JS. 

 Peters.) 



trulation or invagination as in the eggs of the lower 

 phyla, but, like the amnion formation, is arrived at by a 

 short cut i.e., by vacuolization of certain cells. 



After the formation of the embryonic area (germ disc) 

 composed of its three germinal layers, the future develop- 

 mental process consists in a succession of wrinkles and 

 puckers resulting from unequal growth of cells in different 

 locations, a general tendency of the external convex 



