60 



TEXT-BOOK OF EMBRYOLOGY. 



becomes clearly delimited from ectoderm, at least a potential cleft separating 

 the two layers. For a short distance laterally the mesoderm also becomes 



Notocord 



Mesoderm 

 Protentoderm 



Ectoderm 



Yolk entoderm 



Remnant of 

 segmentation cavity 



FIG. 32. Transverse section of embryo of frog (Rana fusca). Bonnet. The section is taken in 

 front of (anterior to) the blastopore. 



delimited from entoderm, but farther laterally it is fused with entoderm 

 (Fig. 32). Then as development proceeds the superficial cells of the yolk 



Neural crest 



Neural canal 



Mesodermal somite = 



Notocord 



Ccelom _ 



Ventral mesoderm < 



Yolk cells 



Ectoderm 



Parietal mesoderm 

 Visceral mesoderm 



Entoderm 



FIG. 33. Transverse section through embryo of frog (Rana fusca). Bonnet. 



entoderm, with which the mesoderm is merged, become differentiated and 

 split off or delaminated and added to the mesoderm. In this manner the 

 mesoderm becomes more extensive until finally it reaches all the way round 



r *-v 



