DEVELOPMENT OF ELASMOBRANCH FISHES. 193 



It is admittedly difficult to prove a negative, and it may 

 still turn out that there are anterior roots of the brain similar 

 to those of the spinal cord ; in the mean time, however, the 

 balance of evidence is in favour of there being none such. This 

 at first sight appears a somewhat startling conclusio-n, but a little 

 consideration shews that it is not seriously opposed to the facts 

 which we know. In the first place it has been shewn by 

 myself* that in Amphioxus (whose vertebrate nature I cannot 

 doubt) only dorsal nerve-roots are present. Yet the nerves of 

 Amphioxus are clearly mixed motor and sensory nerves, and it 

 appears to me far more probable that Amphioxus represents a 

 phase of development in which the nerves had not acquired 

 two roots, rather than one in which the anterior root has been 

 lost. In other words, the condition of the nerves in Amphioxus 

 appears to me to point to the conclusion that lyrimitively the 

 craniospinal nerves of vertebrates were nerves of mixed func- 

 tion with one root only, and that root a dorsal one; and that 

 the present anterior or ventral root is a secondary acquisition. 

 This conclusion is further supported by the fact that the posterior 

 roots develope in point o-f time befo-re the anterior roots. If it 

 be admitted that the vertebrate nerves primitively had only a 

 single root, then the retention of that condition in the brain 

 implies that this became differentiated from the remainder of 

 the nervous system at a very early period before the acquire- 

 ment of anterior nerve-roots, and that these eventually become 

 developed only in the case of spinal nerves, and not in the case 

 of the already highly modified cranial nerves. 



Subsequent Changes of the Nerves. — To simplify my descrip- 

 tion of the subsequent growth of the cranial nerves, I have 

 inserted a short description of their distribution in the adult. 

 This is taken from a dissection of Scy Ilium Stellare, which 

 like other species has some individualities of its own not found 

 in the other Elasmobranchs. For points not touched on in this 

 description I must refer the reader to the more detailed ac- 

 counts of my predecessors, amongst whom may specially be 

 mentioned Stannius^ for Carcharias, Spinax, Raja, Chimsera, 



1 Journal of Anatomy and Physiology, Vol. x. 



2 Ncrvensystcm d. Fische, Eostock, 18i9. 



