1879.] 



and Olfactory Organ of Vertebrates. 



327 



third of these elements — the olfactory nerve proper — is the earliest 

 to be developed, and that the olfactory tract and bulb, when present 

 at all, do not appear till an exceedingly late period of development — 

 a period so late indeed that their ultimate presence affords no ground 

 whatever for separating the olfactory from the other cranial nerves. 



6. The olfactory nerve is a primary nerve, comiparaole to the segmental 

 cranial nerves. 



Certain of the cranial nerves, e.g., the facial and glossopharyngeal, 

 have long been recognized as possessing segmental value. These seg- 

 mental nerves in the early stages of their development possess certain 

 characters in common, which serve to distinguish them sharply from 

 other nerves or branches of nerves ; of these characters the following 

 are the most important : — (1) They appear very early ; (2) they arise 

 (at least in the chick) from the neural ridge 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) and;, finally, they have very definite 

 relations to the segments as indicated by the visceral clefts, each nerve 

 supplying the two sides of a cleft. 



In all these points the olfactory nerve agrees very closely with the 

 segmental nerves : — (1) It appears very early in all the types examined, 

 and in the chick it seems to be one of the very first nerves in the body 

 to be developed ; (2) there is also strong reason for thinking that, in 

 the chick, the olfactory nerve is developed from the neural riclge ;* 



(3) its point of attachment to the brain undergoes a shifting of pre- 

 cisely similar nature to that presented by the segmental nerves ; 



(4) its direction is at right angles to the longitudinal axis of the head, 

 so that were the cranial flexure to be corrected, and the head straight- 

 ened out, the course of the olfactory nerve would be parallel to that of 

 the segmental nerves ; (5) it possesses a ganglionic enlargement at its 

 point of origin from the brain ; (6) and, finally, an attempt will be 

 made iu the second part of this paper to show that it supplies the two 

 sides of a visceral cleft. 



Since, then, the olfactory nerves do not differ embryologically in 

 any material respect from the segmental cranial nerves, they must be 

 regarded as the first or most anterior pair of true segmental cranial 

 nerves. 



The Development of the Olfactory Organ. 

 This will, in the absence of figures, be treated very briefly ; those 



* For a discussion of this point, vide " Quart. Journ. Micro. Science," January, 

 1878, pp. 18, 19. 



