248 



JHE CHICK. 



nent, than the primitive streak. A longitudinal neural groove 

 (Fig. 117, NG) forms along its dorsal surface; this is at first 

 shallow, but rapidly deepens by uprising of its borders as a pair 

 of longitudinal ridges, the neural folds. 



At their hinder ends (Fig. 110), the two neural folds diverge 

 from each other, and embrace between them the anterior end of 

 the primitive streak. In front, the neural folds rapidly increase 



HD 



NF I 



AD 



FIG. 110. A Chick Embryo at the twenty-fourth hour of incubation ; seen from 

 the dorsal surface. Cf, Fig. 109, which shows the relations of an embryo 

 of this age to the blastoderm, x 20. 



AD, margin of area pellucida. HD, head of embryo. MS, mesoblastic somite or 

 protoyertebra. NF, neural fold. WGr, neural groove. PS, primitive streak. VV, 

 vitelline vein. 



in height, the neural groove between them becoming deeper in 

 consequence. About the end of the first day (Fig. 110, XF), the 

 two neural folds meet, in the region of the future hind-brain, 

 converting the open groove into the closed neural tube (cf. 

 Fig. 118); and this closure of the tube rapidly extends both 

 forwards and backwards. 



