Herrick, Brain of the Snake. 167 



ers, and (7) that the cord is not segmented. McClure demon- 

 strated neuromeric segmentation throughout the cord but does 

 not agree with Orr as to the sixth nerve. He notes that the 

 neuromers soon desappear and insists that the fore brain 

 neuromers are homologous with those of other regions. 



Waters decides that the fore brain is composed of [three or] 

 at least two well-marked neuromers. He says: "Of the exis- 

 tence 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 charac- 

 ter. From the second no nerve springs, but it 's directly oppo- 

 site 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 specialized 

 in existing vertebrates, it was originally not so closely identified 

 with the brain itself, but was homologous with the brain itself. 

 From the third no nerve arises, but I think it probable that still 

 lower forms in still ejrlier stages will show some nerve arising 

 at this point." 



(2.) "That the mid-brain consists of two neuromers from 

 which I have every reason to believe the third and fourth nerves 

 take origin, and hence deserve to be- recognized as segmental 

 structures." 



(3.) " That the hind brain consists of si.x neuromeres. In 

 regard to this region I think the observation of McClure and 

 Miss Piatt are sufficiently satisfactory except as regards the 

 origin of the sixtji nerve and the abducens neuromere. This 

 nerve I have found to occupy its theoretical position when its 

 neuromere exists ; when fusion has taken place between the 

 trigeminus and abducens neuromeres, the sixth nerve has been 

 shifted backward toward the seventh and eighth nerves." 



"It seems reasonably certain that the central nervous sys- 

 tem of the primitive Vertebrate form consisted of a series of 

 symmetrical segments, of which those of the neuron held the 

 same relation to the raesoblastic head-segments as did those of 

 the cord to the protovertebrae, 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 



