MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 319 



nal, facial, and other segmental nerves. I have investigated 

 very carefully this point, which I am disposed to regard 

 as of some importance, and find that in the chick, 

 at a time when cranial flexure has attained its maximum 

 development, the angle formed by producing the direction 

 of the olfactory nerve and of the facial or glosso-pharyngeal 

 nerves until they meet, is almost identical with that which 

 measures the amount of cranial flexure ; the angle in 

 either case being about 120°. The course of the olfactory 

 nerve in dogfish embryos is shown in figs. 17 and 18, and 

 in the salmon in figs. 33 and 34. 



5. This is a point of comparatively little importance, 

 inasmuch as in the embryo ganglia, or local accumulations 

 of nerve-cells, appear to be developed in a very irregular 

 manner, and at very various points in the course of the nerves. 

 Still it is a point not altogether destitute of weight, since 

 those cranial nerves which appear for other reasons to have 

 no claim to rank as segmental, are also peculiar in not 

 possessing ganglionic enlargements at or near their roots 

 of origin in the early stages. In the chick these ganglia 

 are shown for the olfactory nerves in figs. 7, 8, and 9 ; for 

 the third nerve in fig. 6 ; and for the trigeminal in fig. 4. In 

 the dogfish the ganglia of the olfactory nerves are shown in 

 figs. 20 and 21. 



6. The discussion of the question whether the olfactory 

 nerve is related to a visceral cleft in the same manner as 

 the segmental nerves are to their respective clefts, will 

 find a more suitable place after the development of the 

 olfactory organ has been considered. 



The distance between the root of the fifth nerve and that 

 of the third is somewhat greater than that between the fifth 

 and the facial, while that between the third and the 

 olfactory is greater still. These facts, which are obviously 

 correlated with the great hypertrophy of the anterior part 

 of the brain, from which the nerves in question spring, can 

 certainly not be used as arguments against the segmental 

 nature of the olfactory nerve. 



Though the olfactory nerve, from the earliest period at 

 which it is recognisable as such, is thus seen to agree with 

 the segmental nerves in all essential characters, it yet 

 presents one or two minor points of difference. In the first 

 place, owing to the close proximity of the forebrain to the 

 nasal pit, the olfactory nerve is shorter than the other cranial 

 nerves at the same age. Secondly, the olfactory appears 

 to lag behind the others in development ; thus, at a time 

 when the other nerves are fil)rillar along the greater part of 



