CHANGES BETWEEN 24 AND 33 HOURS 



S3 



the process. Each neural fold consists of a mesial component 

 which is thickened neural plate ectoderm, and a lateral com- 

 ponent which is unmodified superficial ectoderm (Fig. 26, A), 

 When the neural folds meet in the mid-dorsal line (Fig. 26, B, C) 

 the mesial, neural plate components of the two folds fuse with 

 each other, and the outer layers of unmodified ectoderm also 

 become fused (Fig. 26, D). Thus in the same process the 

 neural groove becomes closed to form the neural tube and the 



prosencephalon 



border of fore-gut 

 subcephalic pocket 

 mesenchyme 



omphalo- 

 mesentericvein 



blood island 



mesencephalon 



notochord 



rhombencephalon 



rhomboidalis 

 Hensen's node 



extra-embryonic 

 vascular plexus 



FIG. 1 8. Dorsal view ( X 14) of entire chick embryo having 8 pairs of somites 

 (about 27-28 hours incubation). 



superficial ectoderm closes over the place formerly occupied by 

 the open neural groove. Shortly after this double fusion the 

 neural tube and the superficial ectoderm become somewhat 

 separated from each other leaving no hint of their former con- 

 tinuity (Fig. 26, JE). 



The Differentiation of the Brain Region. By 27 hours of 

 incubation the anterior part of the neural tube is markedly 

 enlarged as compared with the posterior part. Its thickened 



