GERM LAYERS. 



69 



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- 



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Fig. 52. — Sections of blastodermic vesicle of bat, showing (a) formation of the entoderm and 

 (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 



