MORPHOLOGY OF EYE MUSCLE NERVES 85 



lations, will seem to most students sufficient evidence to demon- 

 strate their neuroblastic derivation. 



The evidence that in early stages of histogenesis the distal fibers 

 of the trochlear are associated with cell clusters or ganglia, as 

 shown by Dohrn and others, affords, as stated above, no support 

 to the cell-chain hypothesis, since these connections with the troch- 

 learis fibers are secondary. Their relations with the trochlear 

 anlage therefore must receive another than a genetic interpre- 

 tation. Such another interpretation is given below. 

 • e. By what means does the increase in length of the neuraxon 

 take place? The histogenesis of the trochlearis affords no evidence 

 in support of the Hensen-Held contention of the differentiation 

 in situ of plasmodesmatous paths in the formation of a nerve an- 

 lage. The loose distal brush of fibers (fig. 49), in which the nerve 

 anlage terminates in early stages, show no such relations to cells 

 or cell processes as Held's position would require. Further- 

 more, while the existence of a loose brush of fibers may readily 

 be interpreted as an advantage to a bundle of neuraxons which 

 effect connection with their terminal organ by a process of free 

 outgrowth, there would appear to be no meaning to the phenom- 

 enon were the Hensen-Held hypothesis correct. As soon as the 

 connection with the myotome is effected by the fibers, they 

 quickly unite into a compact bundle and lose their brush-like 

 form. Therefore, while it is difficult to demonstrate the free- 

 growing terminations of the fibers within the mesenchyma, the 

 positive evidence, as far as it is known, is in harmony with the 

 process theory. The assertion that the trochlear is differen- 

 tiated in situ through the use and incorporation of primary plas- 

 modesmatous paths is wholly unfounded. 



/. What is the source of the cells of the trochlearis? Somatic 

 motor spinal nerves derive their cells from the neural tube by 

 migration along the nerve anlage, possibly from the mesenchyma, 

 and come into close association with cells masses derived from 

 the spinal ganglia. Do the cells associated with the trochlearis 

 anlage have a similar derivation? 



That medullary elements enter the trochlear anlage has been 

 held by Dohrn ( '07) on the basis of evidence similar to that pre- 



