428 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 glosso- 

 pharyngeal nerve alone becomes eventually separated from the 

 succeeding branches. Stannius and Gegenbaur have, as was 

 mentioned above, detected in adult Elasmobranchs 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 probable one, partly from the 

 embryological evidence furnished by my researches, which is 

 clearly opposed to the existence of anterior 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 1 . 

 The similar relations of the apparently 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 embryo- 

 logical evidence it does not however seem possible to decide 

 whether the hypoglossal nerve contains elements only of anterior 

 roots or of both 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. 



