NERVOUS SYSTEir OF THE VEKTEBRATA. 



375 



It is uot known whether the cranial nerves originate before the closure 

 of the neural canal in othtr forms besides the Chick. 



Fig. 270. Tkansvkkse section thkodgh the posteriou pakt of the 

 head of an embryo chick of thirty hours. 

 hh. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochord; x. thickening of 

 hypoblast (possibly a rudiment of the subnotochordal rod); al. throat; ht. heart; 

 pp. body cavity; so. somatic mesoblast ; sf. splanchnic mesoblast; hy. hypoblast. 



The neural crest of the brain is continuous with that of the spinal 

 cord, and on its separation from the central nervous axis forms on 

 each side a commissure, vmiting the posterior cranial nerves with the 

 spinal nerves, and continuous with the commissure connecting together 

 the latter nerves. 



Anteriorly, the neural crest extends as far as the roof of the raid- 

 brain \ The pairs of nerves which undoubtedly grow out from it are the 

 third pair (Marshall), the fifth, the seventh and auditory (as a single 

 root), the glossopharyngeal, and the various elements of the vagus 

 (as separate roots in Elasmobranchii, but as a single root in Aves). 

 Marshall holds that the olfactory nerve probably also originates from 

 this crest. It will however be convenient to deal separately with 

 this nerve, after treating of the other nerves which undoubtedly 

 arise from the neural crest. 



The cranial nerves just enumerated present in their further de- 

 velopment many points of similarity; and the glossopharyngeal nerve, 

 as it develops in Elasmobranchii, may perhaps be taken as typical. 



^ Marshall holds that the neural crest extends in front of the region of the optic 

 vesicle. I have been unable completely to satisfy myself of the correctness of this 

 statement. In my specimens the epiblast along the line of infolding of this part of 

 the roof of the brain is much thickened, but what Marshall represents as a pair of out- 

 growths from it like those of a true nerve (No. 3:4, PI. 11. fig. 6) appears to me in my 

 specimens to be part of the external epiblast ; and I believe that they remain connected 

 with the external epiblast on the complete separation of the brain from it. 



