THE NERVOUS SYSTEM OF AMPHIBIA 295 



for they must be studied under very high magnifications, and the 

 roots come into very close relations with each other as they 

 approach the brain. It seems unquestionable, however, that the 

 more dorsal tract is derived chiefl}' from the dorsal ganglion (a) 

 and that the ventral tract comes chiefly from the ventral gan- 

 glion (b). In the case of the postauditory group, the dorsal 

 tract is derived chiefly from the vagus and the ventral, chiefly 

 from the glossopharyngeal ganglion. It is impossible to say 

 that there is no interchange of fibers between the roots of either 

 set, but if such anastomosis does occur it nuist iinolve only a 

 very few fibers. 



If these lateral line tracts were each composed of fibers from 

 one ganglion exclusively there would be grounds for assuming 

 that they effect a certain degree of localization in the brain with 

 reference to the particular areas with which they are severally 

 connected. Herrick's observation of the mingling of the fibers 

 of the different roots, however, makes it necessary to seek some 

 other explanation for this early differentiation of the lateral line 

 system within the brain. There is nothing in the sense organs 

 to suggest that this has to do with different sensory modes. 

 Its significance, therefore, must in some way have to do with the 

 directness with which stinuili may i)ass to the motor centers. 

 But that this mechanism as it is found in early swimming em- 

 bryos could serve such a function seems incredible, particularly 

 in view of the nature and arrangement of the neurones of the 

 second order in the circuit. It is hoped that the study which 

 is in ]')rogress on the association paths of the brain nia>' offer 

 something towards the solution of this problem. 



In view of Herrick's observations of the bifurcation of the root 

 fibers, it is of interest to note, further, that in the case of the 

 trigeminal and lateral line nerves this takes place very early in 

 development. In the case of the visceral sensory and auditory 

 roots, however, there is little or no evidence of bifurcation of the 

 fibers in the early swimming stage. 



