332 DR. A. MILNES MARSHALL. 



derived by inheritance. Hence it follows either that verte- 

 brates must have acquired or developed an olfactory organ 

 completely de novo, or else that their olfactory organ has 

 been formed by gradual modification of some pre-existing 

 structure with accompanying change of function^. The first 

 of these alternatives may, I think, be at once dismissed as 

 untenable, and then we are left with the second alternative. 



I do not propose to enter here into a detailed discussion 

 of the physiology of smell, but will only remark that what 

 little we do appear to know definitely about it is quite in 

 accordance with the view that smelling is only a modified 

 form of breathing, and that no very violent physiological 

 change would be necessary to convert a gill into an olfactory 

 organ. On the other hand, the sense of smell is something 

 of a totally different nature to sight or hearing ; the essence 

 of these latter consists in the appreciation of the relative 

 wave lengths of undulations conveyed by air or ether ; while 

 smell appears to be due to direct chemical action on the 

 nerve-endings, requiring the presence of free oxygen 

 (Graham) . 



In the first part of this paper I have attempted to show 

 that the cranial nerves afford very definite evidence as to the 

 segmentation of the anterior part of the vertebrate head — 

 evidence, indeed, quite as definite as that which they have 

 long been recognised as affording concerning the hinder part 

 of the head. The second part of the paper has shown that 

 the visceral clefts afford equally definite evidence on the same 

 point. In a former paper" I have called attention to the 

 fact that the early stages of the brain also afford evidence on 

 this point. 



It is a matter of considerable interest that the evidence 

 yielded by these three types of structures respectively, as to 

 the number and situation of the cephalic segments, is identical. 



The brain consists in an early stage of a series of vesicular 

 dilatations separated by slight constrictions ; of these vesicles 

 the most anterior is the forebrain, and the next the mid- 

 brain ; while the succeeding vesicles, which form a series 

 decreasing in size from before backwards, and of which the 

 first two at any rate appear to possess considerable constancy, 

 are spoken of collectively as forming the hindbrain. 



From each of these brain-vesicles a segmental nerve 

 arises : the forebrain gives origin to the olfactory nerve ; the 

 midbrain to the third or oculomotor nerve f from the 



1 Cf. Dolirn, op. cit., •* Princip des Eiinctionswechsels." 

 " 'Journal of Anatomy and Physiology.' vol. xl, p. 510. 

 3 For a full discussion of the reasons which have led me to consider the 



