386 DEVELOPMENT OF ELASMOBRANCH FISHES. 



evidence may be eventually forthcoming. The evidence from 

 my own observation is, so far as it goes, against it. At a time 

 anterior to the outgrowth of the spinal nerves, I have shewn 1 

 that the spinal cord is completely invested 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 2 , 

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

 pletely in accordance with the ordinary views, yet states 3 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 4 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. 



1 Phil. Trans., 1876. [This Edition, No. vm.] 



2 Entwicklungsgeschichte der Unke. 3 Loc. cit. p. 516. 

 4 Archiv fiir Micros. Anat. Vol. xi. 1875. 



