354 



CHORD ATA. 



The embryo is now directly comparable to the gastrula of Amphi- 

 oxus, with the reservation that the hypoblast cells containing yolk are 

 heaped up in the floor of the archenteron. 



The neural tube is now formed in the mid-dorsal line by the up- 

 growth and fusion of two neural folds. These extend backwards 



Fig. 252.— Three Stages in Development of Frog's Egg. 



Small Cells. 



Archicoele. 



„7„^Large Cells 

 with Yolk. 



Commencing 

 Archenteron. 



Blastopore. 



Edge of advan- 

 cing Epiblast. 



Archicoele. 



A, The Modified Blastula. B, Commencing Gastrulation. 

 C, The Modified Gastrula nearly formed. 



to envelop the primitive groove, and convert the blastopore into 

 a neurenteric canal, as in Aniphioxtis. Meanwhile the mid -dorsal 

 cells of the hypoblast become cut off as a solid rod of cells, forming 

 the notochord, and dorso-laterally two sheets of mesoblast are simi- 

 larly cut off" from the hypoblast, their separation taking place slightly 

 before that of the notochord (as in Aniphioxtis). The differentiation 



