EMBRYOLOGY OF TADPOLE AND CHICK 



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region, but lie above the lower pole itself) toward the ventral portion 

 of the blastopore, so that a more or less completely continuous layer of 

 mesoderm is formed between the ectoderm and entoderm. 



In the dorsal region of the blastopore, and extending along the dor- 

 sal axial mass, delamination does not occur as rapidly as in the ventral 

 region. The course of delamination is also modified, here. This modi- 

 fication is no doubt due to the fact that in this dorsal axial region the 

 cells which are to become mesoderm are derived from the cells which 

 invaginated from the outer layer, w r hile in other regions this is not the 

 case. Then, too, it is in this region that the notochord forms, which 

 further complicates matters. 



In fact, cross sections in the region of the blastopore do not show 



B 

 Fig. 325. 



A, B, C, Transverse sections of Amblystoma tadpole. A, through suckers ; 

 B, through optic and olfactory region ; C, through gill region. 



