No. I.] THE PRIMITIVE VERTEBRATE BRAIN. ^q 



its anterior and posterior limits the cells are crowded together 

 and do not enter the adjoining structures. Two nerves are 

 connected with this neuromere of the mid-brain, — the Ociilo- 

 viotor, and Trochlear^ — each of which, according to the recent 

 investigations of Gaskell, conforms to the type of a complete 

 segmental nerve, in that each contains remnants of the primitive 

 sensory elements ; that is, they possess "nerve fibres and groups 

 of ganglion cells corresponding in position, and doubtless also 

 in function, with the nerve fibres and nerve cells of the station- 

 ary ganglia on the afferent root of a spinal nerve." Gaskell 

 suggests that both of these nerves (III. and IV.) are probably 

 complete segmental nerves of the type which Balfour supposes 

 to have been the original type, when mixed motor and sensory 

 roots were the only roots present. I do not consider Gaskell's 

 investigations with respect to the III. and IV. nerves as conclu- 

 sive without further evidence on the subject, but I agree with 

 him, and on entirely different grounds, that the III. and IV. 

 nerves represent two separate segmental nerves. Taking into 

 consideration the size of the mid-brain neuromere in compari- 

 son with the remaining neuromeres of the brain as well as its 

 "neuromeric" characteristics, also the fact that two nerves arise 

 from it, which are probably either two segmental nerves or parts 

 of the same, also the investigations of Kupffer, previously men- 

 tioned, in which he states that he found at least eight segments 

 in the hind and mid-brains of the Trout and Salamander, there 

 can be little doubt left but that the mid-brain originally consisted 

 of at least two neuromeres, and that in all probability the III. 

 and IV. nerves were the segmental nerves of these neuromeres 

 respectively. (See Appendix.^i 



TJie Primitive Fore-brain Neuromeres and their Nerves. 



The optic neuromere (Figs. 7, 8^, 9 ; Nm II) has no connec- 

 tion whatever with any segmental nerve. The optic nerve is 

 undoubtedly secondary in its nature, and is, I believe, consid- 

 ered by all as outside the series of segmental nerves. It seems 

 probable that the primitive segmental nerve of this neuromere 

 degenerated as soon as the vertebrate eye came into existence, 

 the latter requiring a nerve better suited to perform its functions 

 than the nerve which primitively belonged to the neuromere. 



