MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 313 



epithelium lining the bottom of the olfactory pit {olf.). It 

 will be noticed that there is no trace of an olfactory lobe, and 

 that the olfactory nerve presents no ganglionic enlargement 

 at any part of its course. In its general relations, mode of 

 origin, and course, the nerve agrees remarkably closely with 

 the other cranial nerves, while in histological characters it is 

 identical with them. 



In fig. 34 the olfactory nerve (i) is seen in longitudinal 

 and vertical section in a salmon embryo of the same age as 

 that just described. This section shows well the relations of 

 the olfactory nerve to the brain ; it also shows the roots of 

 the optic nerves (ii), the infundibulum (e'w/.), and the 

 trabeculce cranii {tr.). 



It would seem, therefore, that if an olfactory lobe is 

 present at any period in the life of a salmon or trout, it does 

 not make its appearance till very late — so late, indeed, that 

 it could have no claim to be considered as an embryonic 

 structure at all : there is no trace of it at the time of 

 hatching, or, indeed for some days afterwards. 



d. In other vertebrates. — In the AxolotP the olfactory 

 nerve is at first connected with the forebrain, not with the 

 hemispheres. Throughout the whole period of embryonic 

 development it is very short, and in the early stages ex- 

 ceedingly so : it is solid, and agrees completely in histological 

 characters with the other cranial nerves. I have failed to 

 detect an olfactory lobe in any of the stages I have examined; 

 i.e. up to the time of hatching. 



I have also made some observations on the earlier stages of 

 development of the olfactory nerves in the frog, which show 

 that in these stages the nerves are extremely short, and that 

 there is no trace of an olfactory lobe. The resemblance 

 between the frog and axolotl is, as might be expected, 

 exceedingly close. 



In some lizard embryos, for which I am indebted to 

 Mr. Balfour, I have noticed the existence of solid olfactory 

 nerves, with no indication of olfactory lobes, at stages 

 apparently corresponding to the fourth or fifth day of 

 incubation of the chick; and I believe I have succeeded in 

 establishing the existence of olfactory nerves at still earlier 

 stages, before the appearance of the cerebral hemispheres. In 

 the later stages the olfactory lobes are more prominent 

 objects than in the chick. 



I will, in conclusion, quote from Professor Parker the fol- 



* For the oj)porlunitj of investigating the development of the Axololl, I 

 am again indebted to Mr. Edon, of the South Kensington Mui.euai. 



