MORPHOLOGY OF' THE VERTEBRATE OLFACTORY ORGAN, 309 



of a chick embryo of the same age as fig. 36 ; it shows the 

 bundles of nerve fibres, bound together by connective tissue, 

 which together constitute the olfactory nerve ; it shows 

 also how the majority of these bundles are arranged in a 

 circle round the margin of the nerve, while a few smaller 

 bundles lie in the centre. 



Figs. 38 and 39 represent sections taken from the same 

 embryo as the preceding figure. Fig. 38 is a transverse 

 section through the olfactory lobe, and shows the laterally 

 compressed ventricular cavity. Fig. 39 is a transverse section 

 through the anterior part of the hemisphere : the outer wall of 

 the hemisphere is seen to have increased greatly in thickness 

 while the inner wall still remains thin ; so that the ventricle, 

 which is greatly compressed laterally, no longer occupies the 

 centre of the hemisphere, but lies close to its inner side. 



In the adult fowl the olfactory lobe has much the same 

 appearance as at the twelfth day : it is about two and a half 

 millimetres in length, and still contains a central cavity, 

 though this latter is relatively smaller than at the earlier 

 date ; the relations of the olfactory nerve to the lobe are the 

 same as at the twelfth day. 



' In my former paper I stated that there is no trace of an 

 olfactory vesicle at any i^eriod in the life of a chick} This 

 statement my later work now shows to be erroneous ; the 

 chick has an olfactory vesicle, but, as in the dogfish, this 

 vesicle does not appear till an exceedingly late period of 

 development. 



The principal points then, in the development of the 

 olfactory nerves in the chick to which I desire to direct 

 attention are : 



1. The olfactory nerves arise from the forebrain, before 

 the cerebral hemispheres have begun to be developed. 



2. They are at first connected with the dorsal surface of 

 the forebrain, but on the appearance of the hemispheres 

 become driven down to the ventral surface of the brain, and 

 acquire a secondary connection with these latter. 



3. From their earliest appearance the olfactory nerves are 

 solid, and present the same histological characters as the 

 other cranial nerves. 



4. There is not the slightest indication of an olfactory 

 lobe till the latter part of the seventh day of incubation. 



Though these conclusions are in complete accordance with 



my earlier work, they are directly opposed to all other 



accounts with which I am acquainted, with one solitary 



exception, to which I shall refer immediately. As the date 



1 Loc. cit., p. 20. 



VOL. XIX. NEW SER. X 



