THE CRANIAL NERVES. 



263 



outgrowth (Fig. 118, NA), which consists of cells more spherical 

 in shape than those of the surface epiblast, or of the brain- 

 wall, appears first in the region of the mid-brain, but rapidly 

 extends both forwards and backwards : forwards as far as the 

 anterior part of the fore-brain ; backwards along the whole 

 length of the hind-brain, and a certain distance down the 

 spinal cord. 



These outgrowths (Fig. 118, na) are the neural ridges. As 

 they arise before the lips of the neural canal have met, the 

 neural ridges of the two sides are at first completely independent 

 of each other. A few hours later, when closure of the neural 



NA~ 



Fig. 118. — Transverse section across the head of a Chick Embryo at he twenty- 

 fourth hour of incubation, passing through the region of the mid-brain. 

 (Cf. Fig. 110.) x 100. 



B, cavity of the mid-brain. CH, notochord, not yet separated from the hypoblastie 

 wall of the pharynx. E, epiblast. H, hypoblast. NA, neural ridge. RT, commencing 

 heart. TP, pharynx. 



canal is effected, the neural ridges separate completely from the 

 external epiblast, but remain closely attached to the brain ; the 

 ridges of the two sides at the same time coalescing with each 

 other to form a continuous longitudinal band, the neural crest 

 (Fig. 119, in b), extending along the dorsal surface of the brain. 



Almost from its first appearance, and before the neural 

 tube is closed, the neural crest becomes more prominent at 

 certain places. These more prominent parts form paired 

 outgrowths of the crest, and are situated opposite the widest 

 parts of the cerebral vesicles. They are the rudiments of the 

 cranial nerves (Fig. 119, nb), while the intervening narrower 

 parts of the crest (Fig. 120, nb), opposite the constrictions 



