MOEPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 333 



anterior vesicle of the hind brain the fifth or trigeminal nerve 

 arises, and the second hindbrain-vesicle appears to give origin 

 fairly constantly to the seventh or facial nerve ; behind this 

 I have not observed any definite relation between the cranial 

 nerves and the brain-vesicles, which latter become very 

 small and of doubtful constancy as to number and relations. 

 This is, however, a matter of little moment, as the evidence 

 afforded by the cranial nerves themselves and by the visceral 

 clefts in the post- auditory part of the head is of an un- 

 impeachable character. 



As to the clefts, the most anterior is the olfactory cleft ; 

 vide figs. 3, 4, 5, and 6. Next to that is the cleft which, 

 following Prof. Parker, I have spoken of as the lachrymal, 

 i.e. the cleft in front of the maxillary arch : the relations of 

 the third nerve to this cleft are well shown in fig. 6; it only 

 remains to be added that, of the two branches into which the 

 third nerve divides beyond the distal ganglionic swelling 

 shown in fig. 6, one lies at first behind this cleft, the other 

 in front of it. 



We next come to the buccal or mouth cleft, between the 

 maxillary and mandibular arches ; the relations of the fifth 

 nerve to this cleft are well known. Behind this is the 

 hyomandibular or spiracular cleft, supplied by the facial 

 nerve ; and then the branchial clefts, supplied by the glosso- 

 pharyngeal and by the several branches of the vagus. 



These relations are exhibited more clearly in the following 

 table. In the first column are the numbers of the segments, 

 of which, if we count seven branchial clefts, the full ver- 

 tebrate number, there are eleven in all; two of these being 

 prseoral, and eight postoral. The second column contains 

 the brain vesicles, the third the segmental nerves, and the 

 fourth the corresponding visceral clefts. 



third uerve as a segmental nerve, and the fourth and sixth nerves as having 

 DO claim to segmental value, vide 'Quart. Jouru. Micros. Sol.,' Jan., 

 1878, pp. 23 — 28, and pp. 32 — 33. Tlie observations there recorded for 

 the chick I have since verified in the dogfish. 



