130 JOHN BEARD. 4 
posed cleft ; and he further compared the Schneiderian folds of 
the nasal mucous membrane, as Stannius! had previously done, 
to the folds of a gill. 
The facts of development, as stated by Marshall, have been 
here admitted, but at the same time slightly extended, and in 
such a wise that the development of the olfactory nerve and 
organ were shown to agree very closely with the nerve, ganglion, 
and branchial sense organs of any other cranial segmental 
nerve. 
But now as to the relationships of the branches of the olfac- 
tory nerve to the supposed cleft, and as to the nature of the 
branches themselves. 
In its earliest development the olfactory nerve shows nothing 
that can really be homologised with the post-branchial branch 
of acranial nerve. Such a resemblance, when present at all, 
is only existent in much later stages. 
But the post-branchial branch of a cranial nerve, whenever 
developed, is par excellence, concerned with the innervation 
of the gill musculature, and if it contains sensory fibres its main 
portion is motor. There is nothing like a gill musculature, 
even in early stages, connected with the olfactory organ. 
No one has yet described an arterial arch, gill cartilage, or 
musculature, in connection with the supposed nasal visceral 
arch. The Schneiderian folds have indeed, in Elasmobranchii 
and other forms, a certain resemblance to gill folds, but this 
alone would not be sufficient to homologise the two structures, 
and the folding could be more easily explained as brought about 
by the mere physiological need of increased surface. But surely 
it is a great change from a respiratory structure and function 
to a sensory structure and function. A change which, in spite 
of the basis of truth in Dohrn’s law of change of function, 
has not, so far as the writer is aware, been shown to have 
occurred in any other case. True, Dohrn? has recognised a 
! Stannius, ‘Lehrbuch der Vergleichenden Anatomie,’ ii Theil. 
2 Dohrn, “Studien, &.,” No. 2, ‘Mittheil. a. d. Zool. Stat. zu Neapel,’ 
Bad. iii. 
