16-t OKIGIN OF NERVES. 



nerves, I have shewn* that the spinal cord is campletely in- 

 vested by a delicate hyaline membrane. It is difficult to 

 believe that this is pierced by a number of fine processes, which 

 completely escape detection, but which must, nevertheless, be 

 present on the hypothesis of Hensen. 



The facts of the development of nerves in Vertebrates are 

 unquestionably still involved in considerable doubt. It may, 

 I think, be considered as certain, that in Elasmobranchs the 

 roots of the spinal and cranial nerves are outgrowths of the 

 central nervous system. How the final terminations of the 

 nerves are formed is, however, far from being settled. Gotte*^, 

 whose account of the development of the spinal ganglia is com- 

 pletely in accordance with the ordinary views, yet states^ that 

 the growth of the nerve-fibres themselves is a centrifugal one 

 from the ganglia. My own investigations prove that the ganglia 

 have a centrifugal development, and also appear to demonstrate 

 that the nerves themselves near the ganglion have a similar 

 manner of growth. Moreover, the account given in the pre- 

 ceding chapter of the manner in which the nerves become con- 

 nected with the mucous canals of the head, goes far to prove 

 that the whole growth of the nerves is a centrifugal one. 

 The combination of all these converging observations tells 

 strongly in favour of this view. 



On the other hand, Calberla'^ believes that in the tails of 

 larval Amphibians he has seen connective-tissue cells unite with 

 nerve-processes, and become converted into nerves, but he ad- 

 mits that he cannot definitely prove that the axis-cylinder has 

 not a centrifugal growth, while the connective-tissue cells 

 merely become converted into the sheath of the nerve. If 

 Calberla's view be adopted, that the nerves are developed 

 directly out of a chain of originally indifferent cells, each cell 

 of the chain being converted in turn into a section of the nerve, 

 an altogether different origin of nerves from that I have just 

 suggested would seem to be indicated. 



The obvious difficulty, already alluded to, of understanding 

 how it is, according to the generally accepted mode of develop- 

 ment of the spinal nerves, that precisely similar nerve-cells and 



1 PhU. Travf!., 1876. '^ Enttcicklunpsgeschichte der Unke. 



^ Loc. cit. p. 51G. •* Archiv filr Micros. Anat. Vol. xi. 1875. 



