12 Journal of Comparative Neurology. 



the necessity, at least, of the primitive continuity of a nerve 

 fibre no longer maintained. The outgrowth of ventral nerves 

 from the spinal cord was first conjectured by Bidder and 

 Kupfifer who also supposed that the dorsal arose in this manner. 

 Koelliker, Marshall, and especially His have adopted the 

 same view. The last mentioned has given very complete descript- 

 ions of the process of histogenesis and has showed that the 

 sensory fibres are derived from the ganglionic ridge or spinal 

 ganglion rudiments and send fibres in both directions, i.e. into 

 the neuraxis and peripherad. Almost all these authors, includ- 

 ing Sagemehl, His, and Koelliker, have assumed that the out- 

 growing fibres are continuous processes of the central cells, 

 which, accordingly, must attain a prodigious length in order to 

 reach their peripheral end-organs. According to Koelliker and 

 His, cellular elements from the mesenchyme surround the 

 bundles, penetrate between the fibres and, at first sparingly, 

 but afterwards at regular intervals produce the sheath of 

 Schwann. Balfour, on the other hand, remained unshaken in his 

 belief in moniliform adhesions of outgrowing cells which arise in 

 the cord. In vol. H. of his Comparative Embryology he says 

 "The cellular, structure of embryonic nerves is a point on 

 which I should have anticipated that a difference of opinion was 

 impossible, had it not been for the fact that His and Koelliker, 

 following Remak and other older embryologists, absolutely deny 

 the fact. I feel quite sure that no one studying the develop- 

 ment of the nerves in Elasmobranchs with well-preserved 

 specimens could be for a moment doubtful on this point." 

 Dohrn at one time took the same position though we believe he 

 has since receded from it. Hensen's view is, at first sight, 

 quite different. I quote from Hertwig's Embryology as follows. 

 "He [Hensen] opposes the doctrine of the outgrowth of nerve- 

 fibres chiefly from physiological considerations. He can think 

 of no motive which is capable of conducting nerves which 

 grow out of the spinal cord to their proper terminations — which 

 shall cause, for example, the ventral roots always to go to 

 muscles, the dorsal to organs not muscular, and shall prevent 

 confusion taking place between the nerves of the iris and those 



