MORPHOLOGY OF EYE MUSCLE NERVES 69 



somatic motor nerves, since the region through which it grows is 

 filled with a loose mesenchyma before the growing tip of the 

 nerve emerges from the base of the midbrain. Under such con- 

 ditions it is difficult to ascertain that processes of mesenchyma- 

 tous cells make no contribution to the growth of the nerve. The 

 burden of proof, however, falls upon those who claim that the 

 nerve grows by the differentiation, in situ, of plasmodesmatous 

 paths. Such proof has not been given. 



As has been suggested, the presence of a granular envelope 

 around the fibers of the oculomotor is not to be taken as evi- 

 dence of primary protoplasmic paths. Were a primary proto- 

 plasmic path of the diameter of the anlage of the oculomotor 

 present, it could easily be found and traced in the sections. 

 The fact is, however, that in all the stages in which the nerve 

 anlage may be traced to its peripheral termination in the stages 

 before it reaches the myotome, the distal end shows an amoe- 

 boid character, precisely resembling the distal extremity of nerves 

 growing in cover-glass preparations as drawn and described by 

 Harrison, Lewis and Burrows. No direct relation with the proc- 

 esses of adjacent cells nor with a direct plasmodesmatous path 

 extending toward the myotome can be detected. All of the 

 phenomena advanced by Held ('09) in his attempt to reconcile 

 the Hensen with the Kupffer-Bidder-Harrison theory and to 

 establish the assumption of primary plasmodesmatous paths are 

 fully in harmony with the process theory of the free outgrowth 

 of nerve fibers, while the Hensen-Held theory of primary plasmo- 

 desmatous paths finds as little support in the phenomena pre- 

 sented in the histogenesis of the oculomotor as in the growth of 

 spinal somatic motor nerves. 



/. What is the source of origin of the cells of the oculomotor 

 anlage? Dohrn ('91) was the first to affirm the medullary origin 

 of the cells of the oculomotorius anlage and his conclusions have 

 been repeatedly confirmed by later investigators, among them 

 Goronowitsch ('93), C. L. Herrick ('93), Chiarugi ('97), Car- 

 penter ('06) and Gast ('09). Neal ('98) and Held ('09) were 

 unable to find convincing evidence of, but they did not deny, 

 medullary migration into the oculomotor. Dohrn's argument 



