DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 21 



strongly in favour of the view which we have seen to be supported 

 by such very strong a priori evidence, viz. tliat the olfactory 

 nerves are to be viewed as the most anterior pair of true cranial 

 nerves. It is most important to notice that at whatever stage 

 I have definitely recognised the olfactory nerve, its appearance 

 and relations are precisely such as they must inevitably be 

 supposing the nerve really developes in the manner suggested 

 by the a priori evidence ; i. e. in the same manner as the other 

 cranial nerves. 



I have dwelt at length on this subject because my results are 

 directly opposed to the generally, I might almost say, universally 

 received opinion expressed by the leading authorities in our 

 text-books of anatomy and embryology. The olfactory nerve is 

 stated and assumed to be of a totally different nature to the other 

 cranial nerves ; a nature so different that its existence is usually 

 ignored in morphological discussions based on the arrangement 

 and distribution of the cranial nerves. 



The olfactory nerve is generally stated to differ from all the 

 other nerves, except the optic, in two main points : 



1. It is an oiiigrowtk of the brain, and not a mesoblastic 

 structure. This objection need not be considered further ; it 

 having been sufficiently shown^ that all the nerves, lohether cranial 

 or spinal, are outgroivths of the nettral canal. 



2. It is a hollow outgrowth : in other words, an olfactory 

 vesicle is present. That this is not a universal feature is shown 

 by its absence in the chick ; and as it has been wrongly assumed 

 to exist in the chick,- so it may prove to have been wrongly 

 assumed to exist in other vertebrates. 



I will not allude to the literature on the subject further than 

 to notice the account given by Balfour^ of the development of the 

 olfactory organ and nerve in Elasmobranchs. The earliest stage 

 he describes corresponds, so far as one can judge, with a five or 

 six-day chick. At this period there is a pair of " lateral out- 

 growths," or " olfactory lobes " from the cerebral hemispheres : 

 " from the peripheral end of each olfactory lobe a nerve similar 

 in its histological constitution to any other cranial nerve makes 

 its appearance ; " " on the root of this nerve there is a large 

 development of ganglion cells. I have not definitely observed 

 its origin, but have no reason to doubt that it is a direct out- 

 growth from the olfactory lobe, exactly similer in its mode of 

 development to any other nerve of the body." A little later on, 

 " the olfactory lobes have become much more pronounced. The 



^ Vide Balfour, loc. cit., and self, loc. cit. 

 - roster and Balfour, 'Elements of Embryology,' part i, p. 117. 

 ^ ' Journal of Anatomy and Physiologv,' vol. xi, part iii, pp. 444;, 450 

 and 481 ; and Plate XVllI, figs. 3 and 8 a. 



