DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 27 



that it is very far from being a nerve of ''subordinate importance," 

 while, if my account of its development is confirmed, it is a point 

 of some interest to notice that in the chicle the third nerve is the 

 very first nerve to he developed in the lohole body. 



I have shown above that the third nerve resembles the hinder 

 cranial nerves in giving off from a ganglionated distal extremity 

 two branches, of which the posterior is the larger. I would 

 suggest that the branches of the third are strictly comparable 

 with the branches of such a nerve as the glosso-pharyngeal which 

 stand astride of a visceral cleft. Tig. 26 shows that the optic 

 nerve is situated in the angle between these two branches. Whether 

 we have in the eye any modified remnant of a visceral cleft I 

 cannot here discuss ; but it is important to notice that this rela- 

 tion is a very strong argument against the possibility of the optic 

 nerve being in any way a modified segmental nerve. One seg- 

 mental nerve could hardly be so modified as to be, from the verv 

 earliest date at which such an arrangement is recognisable, 

 between the primary branches of another segmental nerve. 



Balfour's important discovery in Elasmobranchs of what he 

 has termed head-cavities^ seems to prove the existence of at 

 least one premandibular segment in the vertebrate head. It is 

 peculiarly unfortunate from my point of view that he has not 

 described the relation of the third nerve to this most anterior or 

 first head- cavity. If the course of the third nerve in the shark 

 is in any way comparable to that in the chick there must be a 

 very intimate relation between these structures. In one of 

 Balfour's figures^ a small bit of nerve, drawn but not described, 

 has the appearance and relations of the third nerve in a four-day 

 chick •? its course is directed straight towards the first head-cavity. 

 This is strong independent evidence in favour of the third nerve 

 being a true cranial segmental nerve. 



I may add that some observations I have recently made on the 

 early stages of development of the salmon, of which I hope shortly 

 to pubhsh a full account, support in a most remarkable and 

 unexpected manner the views advanced above as to the morpho- 

 logical significance of the olfactory and third nerves. 



The fourth nerve. — I have devoted considerable time and 

 attention to attempting to ascertain the mode of development of 

 this nerve, but as yet have completely failed to recognise it in 

 any section at any period up to the end of the fifth day, beyond 

 which date my investigations do not extend. This I greatly 



' ' Journal of Anat. aud Phys.,' vol. xi, part iii, pp. 481, 482. 



^ Loc. cit., plate xviii, fig. 1 a. 

 ii ,' I learn from Mr. Balfour that he believes this to be the third uerve, 

 and believed it to be so at the time of writing his paper. 



