THE NERVOUS SYSTEM. 



499 



etc.). All these ganglia, probably, are innervated by fibers from the white 

 ramus, along whose course they apparently migrated. The axones of their 

 cells pass to visceral structures either in the same segment or, via the longi- 

 tudinal chain, to those of other segments. Some also join the branches of 

 the peripheral spinal nerves (gray ramus). Fibers of the white ramus also pass 

 longitudinally in the chain to vertebral ganglia of other segments. The 

 possibility previously mentioned (p. 493) of a contribution to the sympa- 

 thetic ganglia by cells migrating out along with the ventral roots must be kept in 

 mind. It would seem a priori more probable that these latter would furnish 

 the efferent sympathetic cells, but the efferent cells predominate in the sym- 



Spinal cord r 



Spinal ganglion 



Ventral root 



Mixed spinal nerve 



Myotome . 



Sympathetic ganglion ' 



Fig. 432. — From a transverse section of a chick embryo of 4 J days. Xeumayer. 



pathetic and must thus be regarded as derived partly or wholly from the 

 neural crest which furnishes at least the major part of all the sympathetic 

 cells. 



It seems probable that not all the cells of the neural crest form nerve cells, 

 but some, usually smaller cells, become closely applied to the spinal ganglion 

 cells, forming amphicytes, while others (lemmocytes) wander out along the nerve 

 fibers and become the neurilemma cells, forming the neurilemma. These cells 

 in this case would be quite strictly comparable to the glia cells of the neural 

 tube. According to another view, the neurilemma cells are of mesodermal 

 origin. While this point cannot be considered entirely determined, it seems 

 fairly certain that in some types at least the former view is correct, removal of 

 the. neural crest having resulted in the formation of efferent nerves without 



