BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 125 
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 pre-branchial branches is accounted for. Otherwise the 
development is normal. There is a main stem with primitive 
branchial sense organ and an associated ganglion, the ciliary. 
There are no other branches except the later developing supra- 
branchial nerve (ophth. profund.). This nerve, as elsewhere, 
is developed in connection with the extension forwards of the 
branchial sense organs (fig. 51 eph.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. It affords a better insight 
into the true nature of the olfactory and auditory nerves. 
First SegMentaL Nerve—O.ractory NERveE. 
The olfactory nerve has usually been classed with the audi- 
tory and optic nerves apart from the true segmental cranial 
nerves.'_ Dohrn, in his essay on “ Die Ursprung der Wirbel- 
thiere,” first suggested that the nose was a gill-cleft, and 
Marshall’ very strongly advocated this view as the result of his 
researches on the chick and in Elasmobranchii. He insisted, 
and as I believe with justice, on the segmental nature of the 
olfactory nerve. Huis 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 statement—that 
“the olfactory nerve develops in precisely the same way as the 
cranial (segmental) nerves ;” they arise at first from the upper 
part of the forebrain and gradually shift downwards, acquiring 
by so doing a secondary connection with the cerebral hemi- 
spheres, of which they are at first completely independent ; and 
* Huxley, ‘Anat. of Vertebrates,’ p. 71; Gegenbaur, ‘Elements of Comp. 
Anat.,’ English trans., p. 515; Gotte, ‘ Entwickelungsgesch. d. Unke, &e.’ 
* Marshall, A. M., ‘The Development of the Cranial Nerves in the Chick,” 
“Quart. Journ. Micr. Sci.,’ 1878, p. 23; and also, “Morphology of the 
Vertebrate Olfactory Organ,” ‘Quart. Journ. Mier. Sci.,’ 1879. 
