470 BERTRAM H. WATERS. 
backward, together with the coalescence of the fifth and sixth 
neuromeres, has brought about the condition of affairs which 
may be observed in Amblystoma. That the absence of the 
sixth neuromere in Amblystoma is due to the degeneration 
of the sensory portion of the corresponding nerve I think 
improbable, even as a matter of conjecture as suggested by 
McClure, for except for the great deviation of recent Am- 
phibia, which alone seems to be an insufficient one, there 
appears to be no other valid explanation of the variation. 
The fact remains that the nerve itself, or its representative, is 
present in low forms, and that in those much higher in the 
scale of development both neuromere and nerve have been 
shown to be present, and to occupy almost, if not exactly, their 
theoretical position. 
McClure’s observation in regard to the probable double 
origin of the seventh and eighth nerves seems to me to be well 
founded. 
The auditory vesicle develops rapidly and comparatively 
early, invading all the space lateral to the auditory neuro- 
mere, and might thus easily have caused the eighth nerve to 
shift forward and rise secondarily with the root of the seventh, 
thus accounting for the absence of nerve origin from the 
eighth neuromere. This theoretical evidence seems to me to 
be further enforced by the fact that the seventh, in containing 
both motor and sensory elements, is clearly segmental; while 
the eighth, in being purely sensory, retains thus so much of its 
primitive character, and renders probable its posterior origin, 
its anterior root and its motor fibres having been lost or dif- 
ferentiated. 
In regard to the myelomeres and spinal nerves, I have 
nothing to add to the observations of McClure. 
In the preceding pages I have attempted to contribute a few 
points to the solution of the problem of Vertebrate brain seg- 
mentation. The evidence is far from being conclusive, but 
enough has been done to suggest the results that may finally 
be attained through this method of investigation. I have 
endeavoured to make the following points: 
