MORPHOLOGY OF EYE MUSCLE NERVES 91 



The mere presence of a loose mesenchyma in the region later 

 traversed by the nerve anlage gives no presumption in favor of 

 view of Hensen and Held. The possibility of the utilization of 

 such material in the elongation of the growing nerve may readily 

 be granted, but as yet no demonstration of such use of proto- 

 plasmic bridges has been made. On the contrar}^, it is possible 

 to demonstrate in sections the free growing end of the nerve 

 anlage and to ascertain, on the basis of actual observation, that 

 no plasmodesm or undifferentiated protoplasmic path connects 

 the end of the nerve anlage with the myotome. Such a growing 

 end of the abducens is given in fig. 35, which shows on what 

 slight evidence the hypothesis of the utilization of protoplasmic 

 paths depends. The amoeboid terminations of the nerve fibers 

 (neuraxons) show attenuated connections by means of fine gran- 

 ular threads — as seen in sections — with amoeboid processes of 

 mesenchymatous cells. But such evidence, so far as it permits 

 any inferences at all, favors the opinion that the fine amoeboid 

 processes of the growing tip are derivatives of the nerve anlage 

 itself. For, if the processes of the mesenchymatous cells are 

 genetically related to them, there is equally good ground for 

 thinking that the processes which extend from the end of the 

 nerve anlage in all directions are derivatives of the nerve itself. 

 Harrison's experimental results verify this assumption, since, in 

 his preparations of the living nerve fibers, similar delicate exten- 

 sions of the amoeboid termination of the nerve make their ap- 

 pearance. In such preparations there can be no question of the 

 genetic relations of the fine threads. To call them paths as 

 Held ('09) has done is obviously a misnomer for structures which 

 radiate in all directions. Were a growing structure to depend 

 upon such flimsy paths for the material for its growth, its ex- 

 tension would be neither fast nor far. Furthermore, neither this 

 section nor others, in which the observer may feel confident that 

 he is dealing with the actual termination of the growing nerve 

 anlage, is the relation of the termination to adjacent mesenchy- 

 matous cells such as to suggest the utilization of their substance 

 in the elongation of the nerve. 



