GERM LAYERS. 



65 



the stage corresponding to Fig. 52, c, the embryonic disk of the dog presents on 

 surface view a uniform appearance. 



The first differentiation noticeable in the disk is an opacity at what now 

 becomes defined as the posterior margin of the disk (Fig. 53). As the em- 



FIG. 52. Sections of blastodermic vesicle of bat, showing (a) formation of the entoderm an 3 

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



bryonic disk increases in size a linear opacity appears extending from the 

 opacity at the posterior margin of the disk forward in the medial line to a point 

 somewhat anterior to the center of the disk. The appearance (Fig. 53) is 

 strikingly similar to that of the chick at the same stage (Fig. 46). The posterior 

 opacity corresponds to the crescentic groove, the linear opacity to the primitive 



