TUBULATION OF ORGAN-FORMING AREAS 469 



ectoderm (figs. 221C-E; 234B; 236B). The neural crest material forms a 

 longitudinal strip of cells lying along either side of the dorsal portion of the 

 neural tube. As such, it forms the neural or ganglionic crest. In some verte- 

 brate embryos, as in the elasmobranch fish, Torpedo, and in the urodele, 

 Ambystoma, the cells of the neural crest are derived from the middorsal part 

 of the neural tube immediately after the tube has separated from the skin 

 ectoderm (epidermis). (See fig. 222 A, B.) In other vertebrates, such as the 

 frog, chick, and human, the neural crest material arises from the general area 

 of junction of neural plate and skin ectoderm as fusion of the neural folds is 

 consummated (fig. 234B). 



The neural crest gives origin to ganglionic cells of the dorsal root ganglia 

 of the spinal nerves and the ganglia of cranial or cephalic nerves as described 

 in Chapter 19. Pigment cells also arise from neural crest material and migrate 

 extensively within the body, particularly to the forming derma or skin, peri- 

 toneal cavity, etc., as set forth in Chapter 12. A considerable part of the 

 mesoderm of the head and branchial area arises from neural crest material 

 (fig. 222C-F). (See Chapters 11 and 15.) 



As the neural plate becomes transformed into the neural tube, it undergoes 

 extension and growth. Anteriorly, it grows forward into the cephalic outgrowth, 

 in the trunk region it elongates coincident with the developing trunk, while 

 posteriorly it increases in length and forms a part of the tail outgrowth. 



c. Closure of the Blastopore in Rounded Gastrulae, such as that of 



the Frog 



Neuralization and the infolding of the neural plate cells begins in the frog 

 and other amphibia before the last vestiges of the entoderm and mesoderm 

 have completed their migration to the inside. As mentioned above, the neural 

 folds begin, and fusion of the neural tube is initiated in the anterior trunk 

 region. From this point, completion of the neural tube continues anteriad and 

 posteriad. As the neural tube proceeds in its development caudally, it reaches 

 ultimately the dorsal lip of the now very small blastopore. As the neural tube 

 sinks inward at the dorsal blastoporal lip, the epidermal attachments to the 

 sides of the infolding neural tube fuse in a fashion similar to the fusion of the 

 edges of the neural tube to complete the dorsal epidermal roof. Associated 

 with this epidermal fusion at the dorsal lip of the blastopore is the fusion 

 of the epidermal edges of the very small blastopore. The extreme caudal end 

 of the archenteron or blastoporal canal in this manner is closed off from the 

 outside (fig. 220D), and the posterior end of the archenteron (the future 

 hindgut area), instead of opening to the outside through the blastoporal canal, 

 now opens into the caudal end of the neural tube. In this way, a canal is 

 formed connecting the caudal end of the future hindgut with the neural tube. 

 This neurenteric union is known as the neurenteric canal. 



It is to be observed in connection with the closure of the blastopore and 



