354 CHORDATA. 
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.—TuHRrEE STAGES IN DEVELOPMENT OF FRoc’s Ecc. 
Small Cells. 
Archiccele, 
A 
Large Cells 
with Yolk. 
Archiccele. 
B 
Commencing 
Archenteron. 
Cc 
Blastopore.¢ 
Edge of advan- 
cing Epiblast. ——~ “ 
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 Amphioxus. Meanwhile the ‘mid-dorsal 
cells of the hypoblast become cut off as a sofdd 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 Amphzoxus). The differentiation 
