64 



TEXT-BOOK OF EMBRYOLOGY. 



which give rise to the embryonic structures proper. In 

 other words, the inner cell mass alone is the anlage of 

 the embryo and this at this stage shows no differentiation 

 into germ layers (Fig. 33). 



The initial step in the formation of the two primary 

 germ layers in the mammalian ovum is the differentia- 

 tion and splitting off of the deeper cells of the inner 

 cell mass (Fig. 52, a). These cells are the primitive 

 entoderm and, as a single layer, soon extend around 

 the vesicle until they completely line it. They lie in 

 apposition to the cells of the trophoderm except where 

 separated from them by the remaining cells of the inner 

 cell mass. While the primitive entoderm is extending 

 around the vesicle, vacuolization of the more superficial 

 cells of the inner cell mass takes place (Fig. 52, b) and 

 results in the formation of a cavity between the over- 

 lying trophoderm and the still remaining cells of the 

 inner cell mass. This cavity is known as the amniotic 

 cavity (Fig. 52, c). Its roof is formed by the tropho- 

 derm, w T hile its floor is formed by the remaining cells 

 of the inner cell mass, which have now become arranged 

 in a distinct layer and constitute the embryonic disk 

 (Fig. 52, c). The latter lies directly upon the primary 

 entoderm and constitutes the surface layer of the 

 embryo the ectoderm. Thus at this stage of develop- 

 ment, the roof of the amniotic cavity is composed of 

 cells which are to give rise to extraembryonic structures, 

 or envelopes, while the floor is composed of the two- 

 layered embryo now consisting of ectoderm and ento- 

 derm. Those investigators who attempt to homologize 

 the early differentiation of cells in Mammals and in 

 lower forms, consider this first formed entoderm in 

 Mammals as identical with the yolk entoderm of lower 

 forms and so designate it, although it does not consist 

 of yolk cells. The protentoderm is formed later (p. 66). 



Considering as a specific example gastrulation in 

 the dog, it is to be noted that just before gastrulation 

 begins, the embryonic disk of the dog is essentially 

 similar to that of the bat which has been described 

 (see above), with the exception that in the dog the 

 embryonic disk is not roofed in by the amnion. At 



