DEVELOPMENT OF ELASMOBRANCH FISHES. 205 



paratively late in my investigations that I detected tlie dorsal 

 one. This has clearly the same characters as the dorsal com- 

 missure already described as connecting the roots of all the 

 spinal nerves, and is indeed a direct prolongation of this. It 

 becomes gradually thinner and thinner, and finally ceases to be 

 observable by about the close of stage L. It is of importance 

 as shewing the similarity of the branches of the vagus to 

 the dorsal roots of the spinal nerves. The ventral of the two 

 commissures persists in the adult as the common stem from 

 which all the branches of the vagus successively originate, 

 and is itself continued backwards as the intestinal branch of the 

 vagus. The glossopharyngeal nerve alone becomes eventually 

 separated from the succeeding branches. Stannius and Gegen- 

 baur have, as was mentioned above, detected in adult Elasmo- 

 branchs roots which join the vagus, and which resemble the 

 anterior or ventral roots of spinal nerves; and I have myself 

 described one such root in the adult Scyllium. I have searched 

 for these in my embryos, but without obtaining conclusive results. 

 In the earliest stages I can find no trace of them, but I have 

 detected in stage L one anterior root on debatable border-land, 

 which may conceivably be the root in question, but which I 

 should naturally have put down for the root of a spinal nerve. 

 Are the roots in question to be regarded as proper roots of the 

 vagus, or as ventral roots of spinal nerves whose dorsal roots 

 have been lost? The latter view appears to me the most pro- 

 bable one, partly from the embryological evidence furnished by 

 my researches, which is clearly opposed to the existence of ante- 

 rior roots in the brain, and partly from the condition of these 

 roots in Echinorhinus, in which they join the succeeding spinal 

 nerves and not the vagus \ The similar relations of the ap- 

 parently homologous branch or branches in many Osseous Fish 

 may also be used as an argument for my view. 



If, as seems probable, the roots in question become the 

 hypoglossal nerve, this nerve must be regarded as formed 

 from the anterior roots of one or more spinal nerves. Without 

 embryological evidence it does not however seem possible to 

 decide whether the hypoglossal nerve contains elements only of 

 anterior roots or of botli anterior and posterior roots. 



1 Vide Jackson and Clarke loc. cit. The authors take a different view to 

 that here advocated, and regard the ventral roots described by them as having 

 originally belonged to the vagus. 



