PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 471 
(1) That the fore-brain is composed of at least two well- 
marked neuromeres. 
a. Of the existence of the first I am in doubt. The first 
nerve arises in the same manner, though at an earlier period 
than the other cranial nerves, thus indicating, however slightly, 
its segmental character. 
b. From the second no nerve springs, but it is directly 
opposite to the eye, and the optic diverticula spring from its 
dorsal crest in a manner entirely comparable to the other 
cranial nerves; thus pointing to the conclusion that, though 
highly specialised in existing Vertebrates, it was originally 
not so closely identified with the brain itself, but was homolo- 
gous with the other segmental nerves. 
c. From the third no nerve arises, but I think it probable 
that still lower forms in earlier stages will show some nerve 
arising at this point. 
(2) That the mid-brain consists of two neuromeres, from 
which I have every reason to think the third and fourth 
nerves take origin, and hence deserve to be recognised as seg- 
mental structures. 
(3) That the hind brain consists of six neuromeres. In 
regard to this region I think the observations of McClure and 
Miss Platt are sufficiently satisfactory, except as regards the 
origin of the sixth nerve and the abducens neuromere. This 
nerve I have found to occupy its theoretical position when the 
neuromere exists ; when fusion has taken place between the 
trigeminis and abducens neuromeres, the sixth nerve has been 
shifted backward toward the seventh and eighth nerves. 
It seems reasonably certain that the central nervous system 
of the primitive Vertebrate form consisted of a series of sym- 
metrical segments, of which those of the neuron held the same 
relation to the mesoblastic head segments as did those of the 
cord to the protovertebre, i.e. were intersomitic ; that those 
of the head, ten or eleven in number, gave origin to their 
respective nerves precisely as did those of the cord to the 
spinal nerves; that, in fact, the two regions were perfectly 
homologous in origin, character, and function. Hence from 
