MORPHOLOGY OK THE VERTEBRATE OLFACTORY ORGAN. 3L7 



ov branches of nerves in the following etnbryological 

 characters : 



1. They appear very early. 



2. They arise, at least in the chick, from the neural crest 

 on the mid-dorsal surface of the brain. 



3. Shortly after their appearance their roots undergo a 

 shifting downward of their points of attachment, so that 

 they no longer arise from the dorsal surface, but from the 

 sides of the brain. 



4. They present, at least in their early stages, ganglionic 

 enlargements on or close to their roots of origin. 



5. Their course is at right angles to the longitudinal axis 

 of the head. 



6. Finally, they have very definite relations to the seg- 

 ments of the head, as indicated by the visceral clefts, each 

 nerve supplying the two sides of a cleft. 



The true cranial segmental nerves, such as the facial and 

 the glosso-pharyngeal, agree in presenting all these charac- 

 ters. On the other hand, the non-segmental nerves, or 

 branches of nerves, though they may possess some of the cha- 

 racters above enumerated, yet never present all, and rarely 

 more than one or two. This test suffices to dispose of the 

 claims to segmental rank of the optic, the auditory, the 

 fourth, and sixth nerves, and of the ophthalmic branch of 

 the trigeminal nerve ; while, on the other hand, it serves to 

 demonstrate the segmental value of the third nerve, 



I propose now to apply this test to the olfactory nerve. 



1. In all the types examined the olfactory nerve appears 

 very early. Though the exact date of its first appearance has 

 not been determined with certainty in any case, yet there is no 

 reason for thinking that it arises later than the other cranial 

 nerves. In all the types considered it appears before the 

 cerebral hemispheres.^ In the dogfish it makes its appear- 

 ance earlier than stage k, and in the chick there are strong 

 reasons for thinking that it is "one of the first nerves in the 

 body to appear."^ 



2. I have already attempted elsewhere to prove that in tlie 

 chick the olfactory nerve is developed from the neural crest. ^ 



' Though I have but little doubt on the matter myself, I have not yet 

 succeeded in determining this- point with absolute certainty in the case of 

 the lizard. 



' ' Quarterly Journal Microscopical Science,' Jan., 1878, p. 23. 



^ Loc. cit., pp. 17 — 19. With reference to the extension forward of the 

 neural crest in the chick to the forebrain, Prof. Kolliker suggests (op. 

 cit., pp. 661 — 2), that 1 have been misled by certain folds which appear 

 during closure of the medullary canal, and to which His lias already directed 

 attenliou. With all due respect for Prof. Kolliker's authority, I cauuot 



