DEVELOPMENT OF THE NERVOUS SYSTEM. 



37 



connected with, and some actually enclosed within, the dorsal 

 wall of the tube adjacent to the seam of closure. There results 

 a pair of continuous flaps or ridges of cells connected with the 

 neural tube at its mid-dorsal line and spreading laterally between 

 the tube and the ectoderm (Fig. 15). These are known as the 

 neural ridges or crests and contain the cells which enter into the 

 formation of the spinal ganglia. This condition of the gang- 

 lionic material may be taken as typical for the embr}^os of verte- 

 brates. In fishes a relati\'ely large number of neural crest cells 

 remain within the neural tube and there give rise to sensorv fibers 



W-' 



X^ 



Fig. 16. — Same as Fig. 15, later stage. The cells of the neural crest have mi- 

 grated ventrally to form a spinal ganglion. Processes are seen upon neuroblasts 

 within the tube and upon one spinal ganglion cell. 



which run to the skin. These are the so-called giant ganghon 

 cells of the spinal cord of fishes (Fig. 17). These facts suggest 

 that the cells which give rise to sensory nerve fibers originally 

 lay within the neural tube and have migrated to form the spinal 

 ganglia. Already in Amphioxus, however, a large part of the 

 spinal ganglion cells are situated in the roots of the nerves outside 

 of the spinal cord. In the brain region the neural crest is formed 

 in essentially the same manner as in the trunk, but two very note- 

 worthy facts are to be pointed out. The first is that in the region 

 opposite the ear the neural crest is entirely absent for a short dis- 



