MORPHOLOGY OF EYE MUSCLE NERVES 59 



which form the protoplasmic connections between tube and so- 

 mite and their processes are therefore to be regarded as neuraxons. 



The cell-chain hypothesis of neurogenesis receives no support 

 from the evidence presented in sections of Squalus embryos. 

 The neurofibrillar structure appears in the nerve anlagen before 

 any cells are present in them. 



The growth of a nerve fiber toward its terminal-organ does 

 not involve the use and resorption of primary plasmatic paths 

 but simply the movement and differentiation of the protoplasm 

 of the medullary neuroblast. The most convincing demonstra- 

 tion of the truth of this assertion is afforded by the growth and 

 extension of the processes of Rohon-Beard cells. 



The numerous cells which, in somewhat advanced stages of 

 histogenesis, make their appearance in the ventral nerve anlagen 

 are not of mesenchymatous but of medullary origin; exclusively 

 so in the earlier stages of development. That mesenchymatous 

 cells are added to the growing nerve in more advanced stages 

 to form the connective tissue sheaths seems probable. 



The cells of the motor nerve anlagen have no genetic relations to 

 the neurofibrils or neuraxons. In other words they are not 'nerve 

 cells' in the von Apathy sense, nor do they unite in chains to 

 form the neuraxons or neurofibrils with their sheaths. Whether 

 or not they participate in the formation of the sympathetic is 

 an open question. The evidence on the whole favors, but does 

 not prove, the conclusion that most of the cells of the sympa- 

 thetic have their source in the dorsal ganglia. That the cells 

 of the motor nerve anlagen in Squalus for the most part form 

 the neurilemma cells can be convincingly demonstrated. 



Thus the phenomena of spinal motor nerve histogenesis in 

 Squalus support the Kupffer-Bidder-His theory of nerve histo- 

 genesis, which has recently been strongly confirmed by the bril- 

 liant experiments of Harrison and Lewis. 



