MORPHOLOGY OF EYE MUSCLE NERVES 45 



— they tend to show that however important the plasmodesms 

 may be as guides for the growing nerve, their share in its pro- 

 duction is neghgible. They prove as convincingly as experi- 

 mental data possibly can the logical fallacy of Held's inference, 

 that because the growing tip of the neuraxon is connected with 

 branched plasmodesmatous processes that therefore these proc- 

 esses have a genetic relation to the neuraxon. The significant 

 fact in this connection is that Harrison's preparations show these 

 same processes at the termination of the neuraxons growing 

 freely in salt solutions! The genetic relations are here precisely 

 the opposite . to those inferred — or assumed — by Held in the 

 normal growth of the neuraxon. Even if it be admitted that 

 Held's assertion is correct that the growing and more or less 

 branched point of growth of the nervous substance at its ex- 

 tremity possesses, in advance of its momentarily attained length, 

 a projecting extension which connects (as an undifferentiated 

 plasmatic mass between the termination of the growing nerve 

 and the end-organ), the neuroblast with the terminal-organ, his 

 conclusion, already quoted, that the plasmodesm is utilized in 

 the formation of the nerve is a logical non-sequitur. Nowhere 

 in his excellent monograph does Held give adequate evidence to 

 prove this assertion, and the experimental evidence seems to 

 make the assumption unnecessary. Furthermore, the assertion 

 that the growing termination of the neuraxon is connected by 

 a strand of undifferentiated protoplasm with the end-organ has 

 not been demonstrated. 



As further evidence in favor of the view that neuraxons and 

 neurofibrils are differentiated within plasmodesmatous or proto- 

 plasmic strands, and consequently in conflict with the idea that 

 they grow 'naked' into the vacuolar spaces of the embryo, Held 

 emphasizes the fact that from their first appearance the neuro- 

 fibrils appear surrounded by a layer of granular protoplasm, a 

 relation better seen in cross-sections than in longitudinal ones. 

 On the other hand, von Lenhossek ('06) states, on the basis of 

 observations on the histogenesis of spinal nerves in the chick, 

 that ''first and foremost is it untrue that the young fibers are 

 embedded in protoplasm" but that on the contrary, "the fibers 



