BRANCHIAL SENSE ORGANS OP ICHTHYOPSIDA. 195 



micus profundus ; hence, if the third nerve he the dorsal root of the 

 second segment, then the proximal stem of the nerve is thinner than 

 one of its distal branches. Hence there seems to be no avoiding the 

 conclusion, in which I agree with Krause and Van Wijhe, that the 

 third is not the dorsal root of the ciliary ganglion, but is the ventral 

 root of the second segment. 



Returning to the general schema of the development of the dorsal 

 root of a cranial nerve, it is found that, so far as its development goes, 

 the nerve of the second segment agrees with the schema. In this 

 instance allowance has to be made for the absence of a gill-cleft, and 

 more especially, of a gill-musculature. In this the absence even in 

 the ontogeny of post-branchial and prae-branchial branches is accounted 

 for. Otherwise the development is normal. There is a main stem 

 with primitive branchial sense organs and an associated ganglion, the 

 ciliary. There are no other branches except the later developing 

 supra-branchial nerve (ophth. prof unci.). This nerve, as elsewhere, is 

 developed in connection with the extension forwards of the branchial 

 sense organs (fig. 51, oph. pro.). The reduction which has probably 

 taken place in the nerve of the second segment prepares the way for 

 the recognition and interpretation of the still greater specialisation 

 which the two remaining cranial segmental nerves have undergone ; 

 and affords a better insight into the true nature of the olfactory and 

 auditory nerves. 



First Segmental Nerve — Olfactory Nerve. 

 The olfactory nerve has usually been classed with the auditory and 

 optic nerves apart from the true segmental cranial nerves, 1 Dohrn, 

 in his essay on " Die Ursprung der Wirbelthiere," first suggested that 

 the nose was a gill-cleft, and Marshall 2 very strongly advocated this 

 view as the result of his researches on the chick and in Elasmo- 

 branchii. He insisted, and as I believe with justice, on the segmental 

 nature of the olfactory nerve. His reasons for this view were 

 based on the actual development of the olfactory nerve ; and he 

 states — and so far as my researches go they only confirm his state- 

 ment — that " the olfactory nerves develope in precisely the same way 



1 Huxley, 'Anat. of Vertebrates,' p. 71; Gegenbaur, ' Elements of Comp. Anat.,' English 

 trans., p. 515 ; Gotte, Entwickelungsgesch. d. Unke, &c.' 



2 Marshall, A.M., " The Development of the Cranial Nerves in the Chick," * Quart. Journ. 

 Micr. Sc, 1878, p. 23 ; and also, Morphology of the Vertebrate Olfactory Organ," 'Quart, 

 Journ. Micr. Sc' 1879. 



