18 DK. A. MILNES MARSHALL. 



completely in showing that the nole structures to which the neural 

 ridge gives origi)i are nerves. 



I see, therefore, no escape from the conclusion that the presence 

 of the neural ridge at any part of the brain implies actually or 

 potentially the presence of a nerve, or of a commissure connecting 

 two nerves^ Therefore there must be in the chick some nerve 

 or nerves arising far forwards from the summit of the anterior 

 part of the fore brain, and strictly equivalent, if embryological 

 evidence is at all trustworthy, to the fifth, seventh, or hinder cra- 

 nial nerves, or to the posterior root of a spinal nerve; otherwise 

 the presence of the neural ridge in fig. 6 is utterly unintelligible.^ 



We have now to consider what nerve or nerves these can be. 

 There are only two which can have the slightest claim to be 

 considered in connection with an outgrowth from the anterior 

 part of the fore brain — the optic and the olfactory. Of these 

 we may dismiss the optic nerve for the present, merely noticing 

 that even should the outgrowths from the optic vesicle ever be 

 proved to have any connection with the optic nerve it would not 

 be sufficient to account for the whole of this part of the neural 

 ridge, for the ridge extends forwards in front of the optic vesicle, 

 and almost to the extreme anterior end of the fore brain; so 

 that there would still remain a certain part to account for. This 

 can only be explained by supposing it to be the rudiment of the 

 sole remaining nerve — the olfactory. The last remaining possi- 

 bility, that the extreme anterior part of the neural ridge represents 

 one of the '' intervening parts" destined to shrink up or (?) dis- 

 appear is, I think, completely disposed of by the consideration 

 that in all cases where we are able to trace their ultimate fate 

 these intervening parts become commissures connecting together 

 successive nerves, or rather pairs of nerves. Now, that the 

 extreme anterior part of the neural ridge should be a " commis- 

 sure'^ is simply incomprehensible. 



We thus see that there is a priori evidence of, to my mind, 

 an exceedingly strong nature, that the olfactory nerve is developed 

 from the extreme anterior end of the neural ridge, and that it is 

 strictly comparable to the hinder nerves. Let us now turn to 

 its actual development in the chick. 



At fifty hours there is a small outgrowth of spherical or shghtly 

 fusiform cells, arising on either side from near the top of the 

 fore brain, just at the slight constriction separating it from the 

 optic vesicle of either side. This forms a small process which may 



' This reservation will be fully explained in the sequel. 



'^ The assumption that the chick has preserved merely the earliest stages 

 of development of some ancestral nerve, of which all subsequent traces have 

 been lost, is so unnecessary and unwarranted that I have not tliought it 

 worthy of serious consideration. 



