560 



THE EMBRYOLOGY OF THE FROG 



lines of demarcation between the notochord, mesoderm, and the dor- 

 sal entoderm for some little time ; but sections through the blastopore 

 (while the yolk-plug is still protruding) show the rim of the blastopore 

 to be composed of thick, undifferentiated cells which are a part of the 

 contracted germ-ring. A little laterally, the ectoderm is separated by 

 a narrow space or line which has formed during gastrulation, and the 



D 



Fig. 325. 



D, E, F, Photographs of transverse sections through 12 mm. frog tadpole. 

 The sections are cut diagonally, the left eye (right side of figure) lying more 

 cephalad ; is the most anterior. E immediately posterior to D, but lens was lost 

 sectioning ; F, immediately posterior to middle cut. 



