THE NERVOUS SYSTEM OF AMPHIBIA 301 



words, the entire axone may be thought of as creeping inward 

 past its point of anchorage at the surface of the brain, as an 

 amoeba would creep between obstacles, and as di-agging the 

 perikaryon after it. What the stimulus for such an action may 

 be I shall not 'discuss here. I only wish to emphasize that this 

 is a concrete case of migration of a group of neurones and that 

 the rapid advance caudad of the root fibers within the brain and 

 the concomitant shortening (not wrinkling) of the extracerebral 

 root fibers convinces me that the axones are active factors in 

 this process. 



Another feature of the development of the sensorj^ cranial 

 nerves deserves mention in connection with the question of neuro- 

 biotaxis, namely, the relative rates of development of the nerve 

 trunks and their respective roots. In case of the trigeminal 

 nerve it has been noted in the anatomical part of this paper 

 that the root connections with the brain are well established at 

 a time when no peripheral fibers can be traced to the skin 

 The lateral line primordia, also, are very small and are reached 

 directly by the corresponding ganglia at a time when the latter 

 have relatively long roots connecting with the brain. The most 

 conspicuous case of this kind, however, is the visceral sensory 

 system, the ganglia of which are connected with the placodes 

 and are without peripheral nerves, excepting a few fibers of 

 indefinite distribution from the geniculate ganglion, at a time 

 when the root fibers have already formed a well defined fascic- 

 ulus solitarius which reaches from the facial root to that of the 

 vagus. 



It is apparent, then, that in the development of the sensory 

 cranial nerves the axone runs far ahead of the dendrite. This 

 suggests the inference that, from the point of view of taxes or 

 tropisms, normal peripheral excitation does not stimulate the 

 growth of the axone inward. This is not meant to imply that 

 there can be no nervous processes in these neurones before the 

 dendritic terminals are estabhshed, but that, if there are such 

 processes, they can have no reference to such stimuli as nor- 

 mally act upon the corresponding definitive nerves and have 

 significance in the reactions of the animal. Similar conditions 



