MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 323 



tinuous process, commencing with their earliest appearance, 

 the olfactory nerves are somewhat peculiarly situated. In 

 their early stages, owing to the close proximity of the olfac- 

 tory sacs to the brain, the olfactory nerves are exceptionally 

 short; and, owing to their origins being at first further 

 forward than their insertions, the growth forwards of the 

 face, carrying the olfactory sacs with it, does not at first 

 cause any lengthening of the olfactory nerves. It is not 

 till the sacs get in front of the forebrain that any lengthening 

 is necessary, but no sooner does this occur than a sudden 

 call is made on the olfactory nerves, which, previously quies- 

 cent, now have to commence growing rapidly in length and 

 to continue so doing. 



I would therefore suggest, without wishing to attach too 

 much weight to the suggestion, that this elongation of the 

 olfactory nerve, occurring under these exceptional conditions, 

 may take place partly at the expense of the nerve itself, and 

 partly at the expense of the brain ; and that it is in this way 

 that the olfactory lobe is produced. It is certainly worthy 

 of notice that in the two types — chick and dogfish — in which 

 I have ascertained with precision the date of its first appear- 

 ance, the olfactory vesicle comes into existence just about 

 the time that the most rapid growth of the nose and snout 

 occurs, and consequently just about the time when a sudden 

 and rapid lengthening of the olfactory nerve becomes neces- 

 sary. It is also a significant fact that the olfactory lobe 

 grows very rapidly at first, the nerve itself remaining nearly 

 stationary.- 



The above suggestion renders it easily intelligible that 

 much variety should exist as to the relative lengths of the 

 nervus and tractus olfactorius, even in nearly allied verte- 

 brates; while it is quite possible that, at any rate in some 

 forms, the skeletal elements may have an important share 

 in determining the relative growth of nerve and brain. 



II. The Olfactory Organ. 



a. Development of the olfactory organ. — The consideration 

 of the olfactory nerve having taken up far more space than I 

 had originally anticipated, I shall be compelled to deal with 

 the olfactory organ in a somewhat more summary fashion. 

 The points to which I wish here to call attention are the 

 remarkable resemblances that exist between the olfactory 

 pits and the visceral clefts. As in the first part of the 

 paper I shall deal first with matters of direct observation, 

 and afterwards consider the theoretical side of the subject. 



