MORPHOLOGY OF EYE MUSCLE NERVES 43 



rofibrils is wholly lacking. Proof that they form the neu- 

 rilemma has been given by the writer ('03) and by Carpenter 



('06). 



6. Hoiv does the elongation of the neuraxon take place? 



Does the elongation of a nerve involve simply a migration 

 of protoplasm from the neuroblast cell as the recent experiments 

 of Harrison ('04, '11), Lewis ('06, '07) and Bm-rows ('11) seem 

 to prove? Or does the neuraxon fiber grow by the progressive 

 differentiation of a primary plasmatic connection between tube 

 and myotome as Sedgwick ('94) and Held ('07) have suggested? 



Are we to accept Cajal's evidence of the free termination of the 

 neuraxon fiber in the intercellular spaces of the embryo? Or 

 shall we agree with Held ( '09) that ' ' in the growing nerve we have 

 to do with an organically advancing growth, and not with the 

 pushing through of a cell process?" 



Although the raising of these questions takes us back to the 

 time of von Baer and the beginnings of embryology, it cannot 

 be said that they have been finally answered. Indeed, the prob- 

 lem of how the elongation of the neuraxon process is effected re- 

 mains one of the most vexed questions of histogenesis. 



Cajal ('07) describes the nerve fibers as advancing and grow- 

 ing through the mesoderm, using the cellular interstices. To 

 him the 'Leitzellen' and their anastomosing expansions are always 

 situated on the sides of the axons which, as the result of the 

 disturbing action of the reagents (pyridine, alcohol), attach 

 themselves to, or partially unite with, the mesodermic frame- 

 work. To Held ('09) such evidence of nerve terminations end- 

 ing freely in the intercellular spaces is a result of the rupture of 

 the plasmodesms, with which he affirms the growing tip is nor- 

 mally connected. The growth of the nerve substance he states 

 (p. 274) is correlated with a resorption of a plasmatic path so 

 that the latter passes over into the former and is utilized in its 

 formation. 



Harrison ('01) states that the sensory nerves of Salmo salar 

 possess fine lateral branches ''which might easily be confused 



