THE NERVOUS SYSTEM OF AMPHIBIA 285 



stimulus to act on more distant receptors of arcs with, possibl3^ 

 a lower resistance. The cause of irregularity in the direction of 

 response when responsiveness is low is, therefore, in all proba- 

 bility to be found in the receptor sj'stem and adequate stimu- 

 lation of an arc as a whole rather than in an hypothetical diffuse 

 form of conduction through the central nervous sj'stem. 



2, Respofise to chemical sti?nulatwn 



The question of the existence of a true irritabihty to sub- 

 stances in solution as opposed to their destructive action on the 

 skin has been discussed in Paper I. Bj- spraying small jets of 

 acid upon restricted regions of the embr\'o and by immersing 

 head pieces of transected specimens in acid it has been shown 

 that the skin of the head is like that of the trunk in its sensitive- 

 ness to the action of acid. It appears, therefore, that the defini- 

 tive, ganglionic nerves of the ages under consideration do not 

 differ from the giant ganglion cell system in respect to chemical 

 stimulation. 



3. Response to light 



During the season of 1910 I performed a series of experiments 

 upon the reaction of larvae and embryos of A. opacum and A. 

 punctatum to light. Sunlight, thrown from a mirror through a 

 water jacket and condenser, was employed in such a way as to 

 bring to bear very high illumination upon sharply restricted 

 regions of the animals. As a check on the adequacy of the 

 stimulus a larva of A. opacum was used which was about ten 

 days beyond the age when swimming begins. When intense 

 illumination was brought to bear over the head and eyes of this 

 larva responses were fairly prompt, as the following figures show, 

 the reaction times being indicated in seconds, with the interval 

 between one response and the next stimulus indicated in paren- 

 theses: 5, (15); 10, (15); 8, (20); 8, (30); 8, (20); 3, (30); 8, (30); 

 7, (30) ; 8, (30) ; 8. The responses were always quick, total body 

 movements and swimming away from the stimulus, slightly to one 

 side rather than straight forward from the position when stimu- 



