MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 337 



leads to results almost identical with those at which I have 

 arrived, and affords perhaps the strongest possible confirma- 

 tion of these results. 



Though in the above enumeration of the segmental cranial 

 nerves I have left out the optic nerve, for reasons stated 

 elsewhere,^ it is quite possible that this nerve may ultimately 

 prove to be of segmental value ; in which case it would indi- 

 cate the existence of a cleft between the olfactory and 

 lachrymal cleft. However, I have as yet completely failed 

 to find any evidence of its segmental nature, and must, for 

 the present, regard it as of a totally different nature to any 

 of the other nerves. The case of the auditory nerve is very 

 different, for there can be little doubt that this is to be 

 viewed as merely a specialised branch of the facial.^ 



If the olfactory organs are really a pair of gill slits, then 

 they must have originally communicated with the mouth 

 cavity : and it becomes a matter of considerable interest to 

 determine whether any traces of such a communication 

 still exist. It is quite possible that the grooves which con- 

 nect the nasal sacs with the angles of the mouth in the 

 skate and other Elasmobranchs, and which form the rudi- 

 ments of the posterior narial passages of higher vertebrates, 

 are remnants of this communication. It is difficult to under- 

 stand what function these grooves subserve in Elasmobranchs, 

 and their apparently irregular presence or absence in closely 

 allied genera would well accord with their being disappear- 

 ing rudiments. In connection with this point some obser- 

 vations I have recently made on trout and salmon embryos, 

 though incomplete, appear to possess some interest. 



Fig. 31 is a transverse section through the anterior part 

 of the head of a salmon embryo just about the time of 

 hatching : the section passes through the anterior borders 

 of the olfactory pits (o//".), through the cartilaginous plate 

 formed by the fusion of the two trabeculse {tr.), and, on the 

 ventral side, through a large flattened cavity {al\) ; this cavity 

 is found, by a study of the sections in front of and behind 

 the one figured, to be an anterior prolongation of the buccal 

 cavity, extending forwards in front of the mouth, underlying 

 the olfactory sacs, and reaching almost to the extreme ante- 

 rior end of the head. 



Fig. 33, which has been already described, is a section 

 taken through the head of an embryo of about the same 

 age as that in fig. 31, but a little further back; it shows this 

 same cavity, which, however, is now not completely closed 



* ' Quart. Journ. Micros. Sci.,' January, 1878, pp. 23 — 27. 



* Balfour, op. cit., p. 213, aud self, loc. cit., pp. 34 — 36. 



