DEVELOPMENT OF THE NERVOUS SYSTEM. 51 



from which the fibers have grown out remain in a mass at the 

 side of the spinal cord and form the spinal ganglion. 



It is very important to notice that not all the cells of the neural 

 crest give rise to nerve fibers, but as in the central nervous system, 

 a part of the cells become supporting elements and a part go to 

 form the sheaths of the nerve fibers. It has recently been experi- 

 mentally proved that cells derived from the neural crest migrate 

 out along the bundles of the peripheral processes and form the 



FIG. 26. The ganglion of the IX nerve in Amblystoma punctatum at the time 

 of formation of the central processes. 



FIG. 27. A few cells of the trigeminal ganglion in Amblystoma, punctatum with 

 the fibers of the ramus mandibularis growing out from them. 



neurilemma or cellular sheath of Schwann of the nerve fibers. 

 The fibers of the ventral nerve derive their sheath cells partly in 

 this way and in part from the central nervous system, the cells 

 wandering out directly along the ventral roots. The origin of 

 the cells of this sheath shows their genetic relation with the neu- 

 roglia cells, and consistent with this is the fact that in the cen- 

 tral nervous system the sheath of Schwann is absent, the myelin- 

 ated fibers being surrounded by glia cells. 



