MORPHOLOGY OF EYE MUSCLE NERVES 15 



they are formed by the amoeboid terminations of the neuraxon 

 processes. The protoplasmic and fibrillar relations are precisely 

 analogous with those seen in a Rohon-Beard cell such as is 

 represented in figure 21. The advancing end of the neuroblast 

 cell is protoplasmic and amoeboid, while the more proximal por- 

 tion of the cell has a neurofibrillar structure. There appears 

 no good ground for doubting the analogy between the phenom- 

 ena of growth of the Rohon-Beard cells and those of the medul- 

 lary neuroblasts which form the fibrillar constituents of somatic 

 motor nerves. The evidence of the exogenous derivation of the 

 neurofibrils advanced by Paton seems unquestionably based on 

 incompletely stained preparations. The assumption of the par- 

 ticipation of indifferent cells in the formation of 'paths' for the 

 growing nerve fibers has an equally insecure foundation. 



After the fiber bundle of the dorsal (somatic sensory) nerve 

 unites with that of the somatic motor nerve a cluster of cells 

 makes its appearance median to the mixed bundle at about the 

 level of the dorsal aorta, forming the anlage of the sympathetic 

 ganglion. The derivation of these cells from the neural tube 

 has sometimes been assumed. The majority of opinion has in- 

 cHned to the view that they are derived from the dorsal ganglia. 

 Positive proof of either inference has never been given on the 

 basis of direct observation. Experimental data appear not abso- 

 lutely trustworthy. The present paper makes no contribution 

 toward the solution of this important question. Analogy with 

 the formation of the sympathetic in the head appears to the 

 writer to favor the view that the most of the sympathetic cells 

 have their source in the dorsal ganglia. The problem, however, 

 needs renewed investigation. 



Turning now to a more detailed discussion of some of the 

 mooted points in the problem of nerve histogenesis, we may 

 take up first the important question of the primary connection 

 between nerve and muscle. 



