GERM LAYERS. 79 



entoderm (frog, Fig. {3). One would expect, a priori, that the mesoderm has 

 a similar origin in the higher forms, even if the entoderm has assumed a differ- 

 ent form on account of the fact that the yolk plays little or no part in the process 



Neural 

 fold 



Neural 

 groove 



Ectoderm 



Mesoderm 



Chorda anlage 



Entoderm 



FIG. 64. Transverse section through dorsal part of embryo of frog (Rana fusca). Ziegler. 

 x, Groove indicating evagination to form mesoderm. 



of imagination. As a matter of fact, observations do to a certain extent fulfil 

 the expectation, but, on the other hand, it is not possible to trace the earliest 

 steps in its formation with anything like the degree of certainty with which it 

 can be traced in the lower forms. 



Neural crest 



Neural canal 



Primitive segment 



Notochord 



Coslom 



Ventral mesoderm <. 



Yolk cells 



[ Ectoderm 



Parietal mesoderm 

 Visceral mesoderm 



Entoderm 



FIG. 65. Transverse section through embryo of frog (Rana fusca). Bonnet. 



Taking the chick again as an example, the mesoderm appears first in the 



region of the primitive groove (blastopore). Transverse sections through this 



region show the mesoderm as several layers of small irregular cells interposed 



laterally between the ectoderm and entoderm. In the medial line, or line of the 



6 



