MORPHOLOGY OF EYE MUSCLE NERVES 47 



The intraplasmatic position of the nerve fiber or neurofibrillar 

 bundle, therefore, instead of proving the doctrine of primary 

 plasmatic nerve 'paths,' is equally in harmony with the process 

 theory of nerve development. An examination of Squalus em- 

 bryos shows that, sooner or later, within the plasmatic neuroblast 

 process which effects the first connection between tube and so- 

 mite, the neurofibrillar bundle is differentiated as an axial fiber 

 surrounded by the undifferentiated protoplasm of the neuroblast 

 process. The advancing end of the nerve consists of undiffer- 

 entiated protoplasm and is amoeboid in appearance. The phe- 

 nomena in sections of preserved embryos are essentially identi- 

 cal with those in vitro of the living nerve fiber. The elongation 

 of the neuraxon, therefore, instead of involving the use and in- 

 corporation of primary plasmodesmatous paths into the growing 

 nerve, on the contrary is effected by a movement of the proto- 

 plasm of a neuroblast cell and the endogenous differentiation of 

 the neurofibrils. With especial clearness are the phenomena 

 strikingly shown in the giant cells of Rohon-Beard. 



a. The histogenesis of the cells of Rohon-Beard. The conclusions 

 based upon the study of the histogenesis of spinal somatic motor 

 nerves are greatly strengthened and confirmed by the phenomena 

 presented by the development of the cells of Rohon-Beard. These 

 phenomena are presented in figures 13 to 22. The close pro- 

 pinquity of the nerve center (neural tube) and the terminal-organ 

 (myotome) in the case of the spinal somatic motor nerve makes 

 it very difficult to find wholly convincing evidence of the exten- 

 sion of the neuraxon process, and of the secondary nature of the 

 connection between the two organs. 



■ In the case of the Rohon-Beard cells, however, the neuraxon 

 process, in reaching its peripheral termination, grows into and 

 through spaces where mesenchymatous cells are entirely absent. 

 As shown in figures 13 to 16, the neuraxon process appears 

 primarily as an amoeboid extension of a large, deeply stained 

 cell, lying in the dorsal wall of the neural tube. As the neu- 

 raxon process becomes further extended, its peripheral termina- 

 tion shows many pseudopodia-like extensions. In some cases 



