BEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 17 



after closure of the neural canal ; but some of his figures strongly 

 suggest that^ like the similar structure in the chick, the neural 

 ridge does appear before closure of the neural canal.i 



I propose now to take the several cranial nerves in order, and 

 describe their development as far as I have been able to elu- 

 cidate it. 



The olfactory nerve. — I have already shown" that at the 

 seventy-fifth hour the olfactory nerves bear a very close resem- 

 blance to the other cranial nerves; they are solid, with no indi- 

 cations whatever of an "olfactory vesicle ;" they arise from the 

 fore brain itself, and not from the commencing cerebral hemi- 

 spheres ; their histological structure differs in no appreciable 

 respect from that of the hinder cranial nerves, and their general 

 relations are such as to " strongly suggest that they are strictly 

 comparable" with the other cranial nerves, and that " the olfac- 

 tory nerves are really the first pair of true cranial nerves." I have 

 since studied the early stages of the olfactory nerves very carefully, 

 and my observations, though I can hardly consider them as 

 conclusive, yet tend to strongly confirm my previous suggestion. 



At the twenty-ninth hour the neural ridge extends forwards 

 to the anterior part of the fore brain, in front of the optic 

 vesicles (vide fig. 6, m). There is not the slightest doubt what- 

 ever that the ridge at this part of the brain is in all respects the 

 same structure, having the same morphological significance as 

 that which, further back in the brain, we shall find gives origin 

 to the rudiments of the fifth, seventh, and other cranial nerves 

 (tJ?V?efigs. 9, 10, ni), and in the body to the posterior roots of the 

 spinal nerves. This neural ridge is a perfectly definite struc- 

 ture, and must have the same morphological significance at 

 whatever part of the body it occurs. Its subsequent history in 

 the hinder part of the brain and in the body is made known to 

 us by Balfour's researches on Elasmobranchs, which are con- 

 firmed in many points by my former investigations on the chick. 

 The ridge becomes more prominent at certain points and shrinks 

 up, or (?) disappears completely, at the intervening points. 

 These prominences are the nerve rudiments. As to the inter- 

 vening parts between successive nerve rudiments we know this 

 much definitely, that wherever their subsequent development 

 has been traced they are found to develope into nerves of a 

 commissural nature. Balfour's and my own researches agree 



» Vide 'Phil. Trans.,' vol. 166, part i, plate xvi, figs. Bl, J32, B.3, 

 and Da. In the last figure certain cells marked y are referred to in the 

 description of the figure as "cells left behind on the separation of the 

 external skin from the spinal cord." 



* Loc. cit., pp. 511 — 513, and plate xxi, figs. 13 — 15. 



VOL. XVIIT. NEW SER. B 



