MORPHOLOGY OF EYE MUSCLE NERVES 41 



rofibrillar has not been demonstrated, however, although the 

 analogy of the histogenesis of the Rohon-Beard cells would 

 favor this opinion. 



Although the evidence presented in this paper is not suffi- 

 cient to demonstrate positively that the neurofibrils of somatic 

 motor nerves arise endogenously within those medullary cells 

 which form the protoplasmic connections of tube and myotome, 

 the conclusion that such is the case appears justified on the fol- 

 lowing grounds: 



• 1. The evidence goes to show that medullary cells, by a process 

 of outgrowth, form the first protoplasmic connection of tube 

 and myotome. 



2. In subsequent stages these processes extend ventrad 

 along the median surface of the somite between myotome and 

 sclerotome. 



3. Within these processes which in vomRath preparations may 

 be traced to their connection with medullary cells, the neu- 

 rofibrillae make their appearance. 



4. Since, as shown by Paton ('07) the neurofibrils arise endog- 

 enously within the processes of Rohon-Beard cells, and Besta 

 ( '04) and Held ( '06, '09) have demonstrated a similar histogen- 

 esis in somatic motor neuroblasts of Amniota, analogy would 

 seem to support a similar origin in Selachians. 



Coghill ( '13) has been able to demonstrate that in Amblystoma 

 the neurones of the somatic motor column become well differen- 

 tiated and typically oriented in the spinal cord before the ven- 

 tral roots appear at the corresponding level. The earliest demon- 

 strable root fibers arise as collaterals from these neurones, usually 

 from descending processes. 



5. Is the neuraxon of a soryiatic motor fiber multicellular in 

 origin, or is it the process of a single medullary cell? 



Since the begimiing of the twentieth century the Schwann- 

 Balfour 'cell-chain' theory of nerve development has been sup- 

 ported by Rafaelle ('00), Bethe ('00-'07), Oscar Schultze ('04- '07), 

 Brachet ('05- '07), C'ohn ('05- '07) and possibly von Apathy 

 ( '07) . From its inception the cell-chain hypothesis has depended 



