MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 325 



forty-eighth hour ; a period almost identical, as in the dog- 

 fish, with the opening of the visceral clefts to the exterior. 



In the trout the mode of development of the olfactory pits 

 corresponds very closely with that occurring in the chick and 

 dogfish ; and, as in these two types, their first appearance 

 coincides almost exactly with the opening of the visceral 

 clefts to the exterior. 



The connection between the olfactory nerve and the bot- 

 tom of the olfactory pit is, as already noticed, acquired ex- 

 ceedingly early, very shortly indeed after the appearance of 

 the latter. The condition of the olfactory organ in the dog- 

 fish is shown at stage m in figs. 15, 16, and 19; and at 

 stage o in figs. 17, 18, and 20. In the chick the olfactory 

 organ is shown at the sixty-fourth hour in fig. 2, at the 

 sixty-seventh in fig. 3, and at the ninety-sixth hour in figs. 

 5 and 6. 



Throughout their early stages of development the olfactory 

 organs present a striking resemblance to the visceral clefts, 

 both in form, position, and general relations — a resemblance 

 •which it will be necessary to consider in some detail, inas- 

 much as it has been very generally overlooked hitherto. 



Fig. S represents a longitudinal and vertical section through 

 the head of a chick embryo at the sixty-seventh hour. The 

 section, which is taken in a plane not far from the surface, 

 passes through the hind, raid, and forebrains, through the 

 auditory vesicle (aucL), the eye (o.c), the trigeminal (v), and 

 auditory (viii) nerves, through the anterior visceral clefts and 

 arches, and through the olfactory pit (olf.). The olfactory pit 

 is seen to bear a marked resemblance to the visceral clefts : 

 like them it is situated on the ventral surface of the head ; 

 it is open below ; its axis is at right angles to the longitu- 

 dinal axis of the head, so that were the head straightened 

 out it would be parallel to the clefts ; and its general ap- 

 pearance and relations are such as to strongly suggest the 

 view that it is one of the same series of structures as the 

 visceral clefts. It is indeed separated from the next cleft, 

 that in front of the maxillary arch (Mz.), by an interval 

 somewhat greater than that separating the hinder arches 

 from one another ; but when we consider the enormous 

 hypertrophy which the part of the brain with which it is 

 connected has undergone, this becomes rather an argument 

 in favour of than against the comparison. 



Figs. 4, 5, and 6 are three sections taken from the same 

 embryo, a ninety-six hours' chick. Of these sections, which 

 are taken in a longitudinal vertical plane, that given in fig. 

 4 is the most superficial, that in fig. 6 the deepest of the 



VOL. XIX. NEW SER. Y 



