thi 



THE PERIPHERAL NERVOUS SYSTEM. 127 



of the deeper or nervous layer of the epiblast along the dorsal 

 surface of the embryo. Along the sides of the neural plate, where 

 it passes into the unmodified epiblast of the body wall, and on 

 its inner or deeper surface, opposite to the commencing neural 

 ridges, a pair of longitudinal bands of epiblast appear. These 

 are at first merely the lateral edges of the neural plate, but they 

 soon become separated by lines of demarcation from the neural 

 plate, and rather later from the epiblast along their outer sides. 

 In transverse sections they appear as a pair of small triangular 

 wedges, at the sides of the neural plate, continuous with the 

 epiblast above, but separated by divisional planes, often indis- 

 tinctly marked, from the neural plate on the inner side and 

 the general epiblast on the outer side (c/. Fig. 59, xd). 



These bands of epiblast cells, cut out from the inner or 

 nervous layer of the epiblast, are at first continuous structures, 

 extending the whole length of the embryo. They are spoken of 

 as the neural ridges, and from them the dorsal roots of the 

 spinal nerves, and the majority of the cranial nerves, are derived. 



As the neural folds grow upwards to inclose the neural tube, 

 the neural ridges get carried up with them ; and at a time when 

 the lips of the tube are about to unite, the neural ridges form a 

 pair of longitudinal bands (Fig. 59, nd), projecting outwards on 

 either side from the angle between the external epiblast, EE, 

 and the wall of the neural tube, NS. 



On the closure of the neural tube, the neural ridges separate 

 completely from the external epiblast, and the ridges of the two 

 sides become continuous with each other across the median plane, 

 forming a plate of cells, the neural crest, attached to the dorsal 

 surface of the brain and spinal cord (Fig. 70, nd). 



As the several divisions of the brain are formed, the origi- 

 nally continuous neural ridge of each side becomes discontinuous, 

 growing outwards so as to become more prominent at certain 

 parts of its length, and disappearing in the intervening regions ; 

 it thus becomes broken up into a series of outgrowths, which are 

 the rudiments of the nerves. 



The neural crest, formed by the fusion of the two neural 

 dges, and therefore also the nerves into which the crest becomes 

 t up, are at first connected with the dorsal surface of the 

 neural tube. The permanent attachments of the nerves to the 

 sides of the neural tube are acquired at a later stage, by 



