No. I.] THE PRIMITIVE VERTEBRATE BRAIN. 47 



occupied by the auditory vesicle ; that the glossopharyno-eal 

 neuromere {Nm IX.) gives rise to the IX. nerve ; and that the 

 vagus neuromere {Nvi X) gives rise to the X. nerve. I find 

 in the hind-brain of the Chick an exact correspondence in struc- 

 ture to that of the Lizard ; that is, in the hind-brain of the 

 Chick the auditory vesicles and nerves hold exactly the same 

 relation to their respective neuromeres as the corresponding 

 auditory vesicles and nerves in the hind-brain of the Lizard do 

 to their respective neuromeres. I think this relationship of 

 neuromeres to nerves has been fully described for the Lizard, 

 and I refer the reader to Fig. 6a, which is a camera drawing of 

 the hind-brain of the Chick, that comparisons may be made. 

 We have already seen that the abducens neuromere {Nm VI) 

 is absent in the Newt. I find also, with this one exception, 

 that the remaining neuromeres in the hind-brain of the Newt 

 hold exactly the same relation to the auditory vesicles and 

 nerves as do their corresponding neuromeres in the hind-brains 

 of the Lizard and Chick. 



(See Fig. 4, which is a camera drawing of the hind-brain of 

 the Newt, for comparison with the above-mentioned figures of 

 the Lizard and Chick.) 



It has been shown in the preceding pages that there are two 

 neuromeres in the hind-brain of the Lizard and Chick, which do 

 not give rise to dorsal or sensory roots (abducens neuromere) 

 {Nm VI.) and auditory neuromere {Nvi VIII. ; Figs. 5^, 6a). 

 It seems probable that these two neuromeres must have once 

 been connected with sensory roots when we consider similar 

 structures in the spinal cord and hind-brain and their systematic 

 connection with dorsal roots. The fact that the abducens neu- 

 romere is absent in the Newt may be accounted for in the 

 following manner: namely, that the degeneration of the sensory 

 nerve of this neuromere has resulted in the consequent degener- 

 ation of the neuromere itself. But this is pure conjecture, 

 and then the fact still remains, that these two neuromeres have 

 not degenerated in the Lizard and Chick, both of which are 

 representatives of much higher forms than the Newt Again, 

 the VI. nerve may be the motor element of the primitive seg- 

 mental nerve of this neuromere (abducens neuromere), its 

 sensory branch having become degenerate. The position of its 

 origin, somewhere between the neuromeres, trigeminal and 



