304 THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 



neuraxones may be formed by the cells which early in development appear in the 

 forming nerve (PL XXII, Figs. 6-9). Indeed the presence of these cells has led some 

 embryologists to infer that they have a genetic relation to the nerve fibres. The 

 similarity of dorsal and ventral nerves in early stages and the fact that some of 

 the cells of the dorsal nerve have genetic relation to neuraxones might seem 

 by analogy to afford presumptive evidence that some of the cells of ventral 

 nerves have a similar relation. But it is evident that the presence of cells in the 

 ventral nerves, even if they be of ectodermal origin, does not prove that they form 

 neuraxones, for many, indeed most, of the cells of the dorsal nerves have no genetic 

 relation to the neuraxones. 



I am convinced that the cells of the ventral nerves have nothing to do with the 

 formation of neuraxones for the following reasons: 



First, in the early stages of development when the number of neuraxones in- 

 creases most rapidly (Pis. XXII, XXIII), the cells of the forming nerves are distinctly 

 peripheral with relation to the fibrous portion of the nerves. 



Second, none of the cells of ventral nerves at any stage shows the deeply staining 

 properties characteristic of embryonic ganglion cells in the dorsal ganglia. The cells 

 seen in ventral nerves are without exception vacuolated, granular, and lightly stained. 



Third, the cells of the forming ventral nerves show no change in shape or size 

 correlated with the growth of the neuraxones, but such changes are always seen in the 

 nerve-forming cells of the dorsal ganglion. The cells of the ventral nerves lie in the 

 periphery of the fibrillar portion of the nerves with their long axes perpendicular to the 

 fibres, an anomalous position for cells supposed to be in process of nerve formation. 



Fourth, in no instance was anything resembling a neuraxon, either formed or 

 in the process of formation, to be found within the cytoplasm of the cells of the form- 

 ing ventral nerves. 



If the cells of ventral nerves in Squalus do not form neuraxones, what is their 

 fate? They form the neurilemma. Their transformation from simple epithelial 

 cells in the periphery of the forming nerves to spindle-shaped cells lying among and 

 around the neuraxones, and finally to the tubular structures of the adult neurilemma, 

 have been followed and are shown in the figures that accompany this paper 

 (Pis. XXII-XXIV). That some of these cells may also contribute to the formation 

 of the connective-tissue sheaths of the nerves and to the sympathetic is not improbable- 



b. What is the origin of the neurilemma? My observations confirm the conclu- 

 sion of von Kupffer ('94, '95) , Hoffmann ('97), and Harrison (:01), that the cells on the 

 forming ventral nerves are emigrated medullary elements and therefore of ectodermal 

 origin like the cells of dorsal nerves. This conclusion is based on the following facts : 



