THE NERVOUS SYSTEM OF AMPHIBIA 299 



the brain and pressure exerted by the dilation of the auditory 

 vesicle in its growth. 



In my discussion of the hyomandibular nerve of Amphibia 

 ('02, pp. 264-271) I pointed out differences between the Anura 

 and Urodela which then seemed to me to warrant the hypothesis 

 that the ramus alveolaris VII of the latter was a pretrematic 

 nerve. The essential basis for that hypothesis was the apparent 

 prebranchial position of the nerve in relatively advanced larvae 

 "anteriorly of the deep pharyngeal evagination which represents 

 the embryonic spiracular cleft" ('02, p. 228). This position, how- 

 ever, must be secondary, for, as described in this paper, the first 

 nerve that grows out from the geniculate ganglion enters the 

 truncus hyomandibularis and passes behind the ectodermal-ento- 

 dermal contact of the spiracular pouch. This position must be 

 primary, as Emmel found it to be for the chorda tympani in the 

 development of ^licrotus ('04), and the nerve must undergo very 

 much the same kind of a shifting in position from an actual 

 postbranchial position to an apparent prebranchial position just 

 as the chorda tympani does in mammals, according to Emmel's 

 descriptions. Contrary to my earlier interpretation, therefore, 

 the r. alveolaris MI of Urodela should be regarded as homolo- 

 gous with the chorda tympani of manunals. 



6. Xeurobiotaxis 



In numerous contributions (cited in his report which is men- 

 tioned in the bibliography appended to the present paper) 

 Kappers has presented an interesting array of data to show that 

 the definitive position of the perikarya of motor neurones in the 

 brain of vertebrates is determined by "a process of taxis or 

 tropism occurring under normal conditions of nervous action, 

 that is, under the influence of reception and propagation of 

 stimuU." This process he designates neurobiotaxis. It involves 

 the conception of a bodily migration of the motor neurones 

 towards the chief source of stunulation by the afferent neurones 

 or through the tracts of the brain. A discussion of this concep- 

 tion as regards motor neurones does not belong in this paper on 



