MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 321 



between the olfiictory and the other cranial nerves. A structure 

 that does not make the slightest appearance till the seventh 

 day in the chick and stage o in the dogfish ; a structure 

 that, in the chick, does not appear till long after the nerves 

 have acquired their connection with the cerebral hemispheres, 

 a connection which I must repeat is a purely secondary 

 one, and not, as Prof. Huxley would have it, primary ; such 

 a structure can hardly be deemed of sufficient morpho- 

 logical importance to outweigh the very obvious and 

 striking resemblances between the olfactory and the other 

 cranial nerves to which I have already referred. 



Again, if my observations are correct, the olfactory nerves 

 cannot be said to appear *' very late ;" while, if I may 

 assume that I have fairly disposed of the olfactory vesicle 

 difficulty, 1 fail to see what are the "^ peculiar relations" of 

 the olfactory nerve that would justify its exclusion " from 

 the category of segmental cranial nerves." 



The condition of the central nervous system appears to me 

 to afford evidence of some value in favour of the ses'mental 

 nature of the olfactory nerve. There is certainly no obvious 

 reason why the anterior cerebral vesicle, or forebrain, of the 

 embryo should be cojisidered to be of a different nature 

 to the middle cerebral vesicle, or midbrain, or to any one 

 of the vesicles of the hindbrain. The early embryonic 

 stages afford no evidence whatever of a break of any kind 

 between the fore and midbrains ; and, if the nerves arising 

 from the mid and hindbrains have segmental value, there 

 is surely a presumption in favour of tlie nerve that takes 

 its origin in the forebrain having a similar and equivalent 

 value j a presumption greatly increased in probability by 

 the close similarity between the early stages of development 

 of that nerve and of the nerves arising further back in the 

 brain. 



It still remains to be considered what is the morphological 

 import of the olfactory lobe or vesicle ; but this is a question 

 to which, in the present state of our knowledge, any answer 

 that may be given must partake very largely of a specu- 

 lative nature. The principal facts we have to guide us 

 appear to be : 



1. The very late appearance of the olfactory lobe. 



2. The fact that though the olfactory lobe is obviously 

 connected with the root of origin of the olfactory nerve, yet 

 it has no relation to the original position of the root of 

 the nerve, and does not appear till this root has acquired 

 a new, and purely secondary position. 



3. The fact that the olfactory lobe does not appear at 



