41 6 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 conclusion, but a little con- 

 sideration shews that it is not seriously opposed to the facts 

 which we know. In the first place it has been shewn by myself 1 

 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 primitively the crania-spinal 

 nerves of vertebrates were nerves of mixed function 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 

 of time before 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 acquirement 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. 



and of Jackson and Clarke on Echinorhinus, Journal of Anatomy and Physiology, 

 Vol. X. These morphologists identify certain roots springing from the medulla below 

 and behind the main roots of the vagus as true anterior roots of this nerve. The 

 existence of these roots is not open to question, but without asserting that it is im- 

 possible for me to have failed to detect such roots had they been present in the embryo, 

 I think I may maintain if these anterior roots are not present in the embryo, their 

 identification as vagus roots must be abandoned ; and they must be regarded as be- 

 longing to spinal nerves. This point is more fully spoken of at p. 428. 

 1 Journal of Anatomy and Physiology, Vol. x. [This Edition, No. IX.] 



