48 McCLURE. [Vol. IV. 



facial, may give credence to this view. It is also possible that 

 the VI. nerve is a motor branch of the V. or VII. nerves : the 

 persistence of the VI. nerve and the absence of the abducens 

 neiiromere in the Newt certainly imply as much. 



Most of the early investigators are agreed concerning the 

 origin of the VII. and VIII. nerves from a primitively single 

 trunk, based on the relations of the VII. and VIII. in Mammals. 

 The opposed view of their separate nature has been steadily 

 gaining ground, and I think at present the latter theory has the 

 greater number of supporters. The double nature of these 

 nerves certainly suggests the probability that they were prim- 

 itively of separate origin, and the following theoretical evidence 

 may throw some light on this theory. The auditory neuromere 

 {Nui VIII) has no nerve connected with it, and it is situated 

 posterior and adjacent to the facial neuromere {Nni VII), which 

 gives rise to the VII. and VIII. nerves. (Awz VII. , Nm VIII.; 

 Figs. 4, 5^, 6a) 



The auditory vesicle (on each side of the brain) is situated in 

 the space lateral to the auditory neuromere, {and; Figs. 4, 5^, 

 da), but the dimensions of the vesicle occupy so much of this 

 lateral space, that the space left between the neuromere and 

 the vesicle is very narrow ; so narrow, in fact, that a nerve aris- 

 ing from the neuromere could not possibly obtain a growth in it 

 sufficient to perform the functions required of the auditory 

 nerve. Thus it is possible that the VIII. nerve may have been 

 primitively connected with the auditory neuromere before the 

 auditory vesicle became so prominent, and that the gradual 

 growth of the vesicle has pushed it from its original position 

 anteriorly into the facial neuromere, where the fusion of its 

 root with that of the VII. nerve has taken place. 



Mid-brain Neiiromcres {Nni III and Nm IV). 



In the Nczvt, Lizard, and Chick the mid-brain has the appear- 

 ance of being an enlarged neuromere, larger than any one of the 

 remaining neuromeres of the brain, but equal in size to about 

 three or four of the first five neuromeres in the hind-brain, and 

 not quite as large as the three neuromeres in the primitive fore- 

 brain. Its cell structure is radial, but its nuclear arrangement 

 does not conform to that of a typical neuromere, except that at 



