MOEPHOLOGY OF EYE MUSCLE NERVES 53 



sensory ganglia, no such changes appear in the cells of the so- 

 matic motor nerves. 



4. While the long axes of the neuroblasts of the sensory nerves 

 correspond, and are parallel with the direction of growth of the 

 neuraxons, no such relation is seen in the cells of the somatic 

 motor nerves in those early stages when the fibers increase in 

 number most rapidly. 



5. The neurofibrils do not lie within, nor are they directly con- 

 nected with, the protoplasm of the cells of the somatic motor 

 nerve anlagen. The cells in the earUer stages of histogenesis 

 are clearly peripheral to the neurofibrillae. On the basis of such 

 considerations, it appears extremely doubtful if the cells of so- 

 matic motor nerve anlagen have genetic relations to the nerve 

 fibers. The cells must have some other fate. 



On the other hand, Cajal ('08) thinks that he has been able 

 to discover nerve-forming cells in a bipolar phase in the somatic 

 motor nerves of the chick. 



9. What is the histogenesis of the neurilemma? 



A second, and practically unanimous opinion is that the cells 

 of embryonic nerves form the neurilemma sheaths. Indeed, this 

 has beien the opinion of investigators, whatever their creed as 

 to the histogenesis of nerve. According to supporters of the 

 cell-chain theory (e.g., Dohrn, '91), that portion of the peripheral 

 protoplasm of the nerve cells which does not enter into the 

 formation of the neuraxon becomes the neurilemma sheath. The 

 same opinion is shared by supporters of the process and of the 

 plasmodesm theory. Difference of opinion centers chiefly about 

 the problem of the origin of the cells. 



Harrison ('06), on experimental grounds, concluded that the 

 neurilemma cells of the frog are exclusively derived from the' 

 neural crest and this opinion is shared by Held ('09) who thinks 

 that the neurilemma cells have a similar origin in Axolotl and 

 Triton. According to Held the neurilemma cells are peripher- 

 ally emigrated glia cells which have secondarily followed neu- 

 rofibrillar tracts aheady laid down. Their source varies in dif- 

 ferent vertebrates. In Petromyzon and the Selachii the source 



