THE NERVOUS SYSTEM OF AMPHIBIA 197 



with the degree of concentration of the acid, that is to say, the 

 intensity of the response tends to vary directly with the destruc- 

 tive action of the stimulating agent while the duration of the 

 response tends to vary inversely with the destructive action. 



Myograms made by this method on certain embryos under 

 the action of HCl n/1000 showed no positive evidence of stimula- 

 tion ; yet immersion in the same solution for a period of fifteen 

 minutes caused pronounced disintegration of the skin. Pro- 

 longed study with these quantitative methods has given a mass 

 of unquestionable evidence in favor of the conclusion that the 

 action of hydrochloric acid upon the skin of amphibian embryos 

 cannot be regarded as a normal stimulation of nerve endings 

 or sensory cells of any sort. 



Various observations by other methods have confirmed this 

 conclusion. If, for instance, as observed also independently 

 by a student in my laboratory, Mr. M. W. Shipley, an embryo 

 of Amblystoma is immersed in a dilution of HCl which does not 

 excite movement, and is, after a brief period, transferred again to 

 the original medium of pond water, it is, by this last immersion, 

 excited to a long series of convulsive movements. This character- 

 istic of the action of HCl on the skin was studied closely, in one 

 case, upon 40 embryos of Amblystoma in the coiled-reaction 

 stage. In this experiment each embryo was first observed for 

 one minute in a dish of pond water and its movements noted. It 

 was then placed in HCl n/1000 and observed for the same length 

 of time. Immediately following this it was replaced in the pond 

 water from which it was originally taken. Through the whole 

 process every movement was recorded. As a result of this ex- 

 periment upon fort}^ embryos the following conclusions were 

 drawn: (1) There was no great variation among the embryos 

 in the degree of normal activity in pond water. (2) There was 

 great variation among the embryos in irritability to the acid. 

 (3) There was great variation among the individuals in their 

 irritability to pond water after the bath in HCl, and there is a 

 distinct correlation between irritability to the acid and the sub- 

 sequent irritability to pond water. (4) There was, on the whole, 

 much greater activity in the pond water following the bath in 



