164 PROFESSOR MARSHALL. 



widely continuous with one another across the top of the brain; 

 that by growth of the mid-dorsal roof of the brain the two nerves 

 get separated from one another; that the root acquires a secon- 

 dary attachment to the side of the brain (fig. 3, VII), but that, 

 unlike the other cranial or spinal nerves, it retains the primary as well 

 as the secondary root throughout life. In this respect the seventh is, with 

 the possible exception of the fourth, the most primitive nerve in the 

 body, inasmuch as it exists throughout life in a condition which is 

 only a transitory one in all the other nerves. However unexpected 

 this point may be, I cannot but think that it is one of the greatest 

 importance in the determination of any question concerning the mor- 

 phology of the cranial and spinal nerves. 



The seventh being a very primitive nerve, there is strong a priori 

 reason for thinking that the sixth nerve, which we have seen reason 

 for grouping with the seventh, is also of a primitive nature, and it is 

 clear that on. the second hypothesis such is the case, the complete 

 independence of the sixth nerve being merely the retention of a primi- 

 tive character, while its limited and special distribution to muscles 

 not present in Amphioxus affords a very possible explanation of its 

 appearance in higher vertebrates. On the first hypothesis, on the 

 other hand, the sixth nerve would be, not a root which had retained 

 its primitive independence of the seventh, but a root which had as a per- 

 fectly exceptional occurrence acquired independence, a view directly 

 contradicted by the primitive condition of the seventh itself. 



It must surely be regarded as a very significant fact that a trans- 

 verse section through the hind-brain of either an embryo or adult Elas- 

 mobranch passing through the roots of the sixth and seventh nerves 

 (fig. 7) agrees absolutely in all essential points with a section at an 

 early embryonic stage through the roots of a spinal nerve in the same 

 animal, i.e. that a condition which is transitory in the case of 

 the spinal nerves is permanently retained in the case of the sixth 

 and seventh nerves. This fact, which is the strongest possible argument 

 in favour of the second hypothesis, clearly directly contradicts the first. 

 If the doctrine that the cranial nerves are more primitive than the 

 spinal appear at first sight paradoxical, 1 I would point out that there 



1 1 have myself on a former occasion both felt and urged this objection (" Head Cavitie3 

 of Elasmobranchs," Quart. Journ. of Miscros. Science, Jan. 1881, p. 91). Further investi- 

 gation has convinced me that I was then wrong, and that Balfour was right in considering 

 (Elasmobranch Fishes, p. 193) the cranial nerves as more primitive than the spinal, though 

 do not agree with his conclusion that the cranial nerves hare no anterior roots. 



