DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 23 



the spinal nerves, or of the vagus or glosso-pharyngeal ; the 

 olfactory nerve, in fact, being one of the first nerves in the body 

 to appear. 



In conclusion, I venture to hope that my account of the 

 development of the olfactory nerve may throw some light on 

 that favourite problem of morphologists — the composition of 

 the vertebrate head ; and this at what has hitherto proved its 

 most impregnable point — the extreme anterior part of the head. 



If the olfactory nerve really prove to be, as suggested above, 

 the most anterior true cranial nerve, it must be intimately con- 

 nected with the most anterior cranial segment. Each of the 

 other cranial nerves, as is well known, forks over a visceral cleft. 

 Have we any trace of a cleft in connection with the olfactory 

 nerve ? In connection with this point, which I hope to discuss 

 fully in a future paper, I will here only mention that while 

 working at Dr. Dohrn's zoological station at Naples in the 

 spring of 1875 I devoted considerable attention, at Dr. Dohrn's 

 suggestion, to the olfactory organ of fish ; one of the results of my 

 investigations being a firm conviction, based solely on anatomical 

 and histological grounds, that there was some very close relation 

 between the olfactory organ and the gills of fish. At that 

 time, however, the ordinary descriptions of the development of 

 the olfactory nerve appeared almost conclusive against such a 

 view. 



The optic 7ierve. — About this I have very little to say. The 

 existence of the neural ridge along the whole length of the optic 

 lobes at an early period is a point of considerable interest. I 

 have not succeeded in tracing the ultimate fate of this ridge, 

 and indeed have not recognised it in specimens later than thirty- 

 six. hours ; I have failed also to prove that any part of this ridge 

 is any way concerned in the development of the eye. 



It would appear, therefore, that the optic nerve is in no way 

 comparable to the other cranial nerves, from all of which it 

 would be sharply distinguished as well by its origin as a hollow 

 diverticulum of the brain, as by its perfect independence of the 

 neural ridge. In view, however, of the extremely complicated 

 character of the vertebrate eye it is quite conceivable that the 

 optic nerve, or some part of it, may have been primitively of 

 the same nature as the other cranial nerves, and that all vestiges 

 of this similarity have been gradually effaced during the evolu- 

 tion and perfection of the organ. 



The third nerve. — In the adult the third nerve arises from the 

 ventral surface of the mid brain close to the median line. 



