88 TEXT-BOOK OF EMBRYOLOGY. 



lower forms. The inner cell mass is destined to form the embryo proper 

 while the covering layer gives rise to certain accessory structures. The 

 covering layer receives the name trophoderm (trophoblast) because in sub- 

 sequent development the nutriment for and waste from the growing embryo 

 must pass through it. 



FIG. 59. Sections of blastodermic vesicle of bat, showing (a) formation of the entoderm and 

 (b and c] of the amniotic cavity, van Beneden. 



v/ Ectoderm and Entoderm. In the lower forms described in the earlier 

 chapters the two primary germ layers arose through the process of gas- 

 trulation. In mammals generally gastrulation is masked under highly 

 modified processes which are probably determined by the peculiarities of 

 cleavage and blastocyst formation in the mammalian ovum. It is usually 

 difficult to recognize any phenomenon in germ layer formation in mammals 



