22 



DR. A. MILNES MARSHALL. 



root of the olfactory nerve is now very tliick, and the ganglion 

 cells it contains are directly prolonged into the ganglionic portions 

 of the olfactory nerve." Balfour also states that " no rudiment 

 of an olfactory nerve" appears till after the olfactory pit has 

 been developed for some time, and its lining membrane raised 

 into the Schneiderian folds. Finally, in discussing the bearing 

 of the cranial nerves on the question of the segmentation of the 

 head, he says : " Although it has been shown above that the 

 olfactory nerve develops like the other nerves as an outgrowth 

 from the brain, yet its very late appearance and 2^eculiar rela- 

 tions are, at least for the present, to my mind sufficient grounds 

 for excluding it from the category of segmental cranial nerves."^ 

 But little comparison is possible between our two accounts, 

 since Balfour^s only begins at about the point at which mine leaves 

 off. I would notice, however, that if '^ the olfactory nerve of 

 Elasmobranchs is a direct outgrowth from the olfactory lobe," 

 then, instead of being " exactly similar in its mode of develop- 

 ment to any other nerve in the body," it differs fundamentally 

 from all the other nerves in the body with which we can com- 

 pare it^ in the following important points : 



1. It does not arise from the mid dorsal line of the neural 

 canal. 



2. The two nerves are not at first directly continuous with one 

 another across the top of the neural canal. 



3. There is no shifting downwards of the point of attachment 

 of the nerve to the neural canal. 



4. The date of first appearance is very much later than that of 

 the other nerves. 



A good deal of the apparent contradiction between our ac- 

 counts may very possibly be removed when we know the limits of 

 extension of the neural ridge in Elasmobranchs, and the earliest 

 stayes of development of their olfactory nerves. On one point 

 there appears to be direct opposition — the presence of an olfactory 

 lobe, i.e. a hollow diverticulum of the cerebral hemisphere from 

 which the olfactory nerve arises, and even here we must wait 

 for further evidence, for in a fifty-hours' chick there is a well- 

 marked olfactory nerve before the cerebral hemispheres have begun 

 to appear, and therefore long before an olfactory lobe could 

 vossibly be present. 



As to the '^ very late appearance " of the olfactory nerve, 

 if my description is correct, the first rudiment of the olfactory 

 nerve appears in the chick before there is any trace of any of 



' Loc. cit., p. 481. The italics are mine. 



2 Tliis reservation is to exclude tiie optic nerve, the anterior roots of the 

 spinal nerves, and any cranial nerve tliat may prove to resemble these latter 

 in their mode of development. 



