196 G. E. COGHILL 



As a result of experiments along the line indicated above, the 

 leading question in my experimentation with acid took this form: 

 can a concentration of acid be found that will stimulate and not 

 destroy the skin; and, if so, is there anything in the nature of 

 the response that differentiates normal stimulation from destruc- 

 tive actions by substances in solution? 



As a means of studying this aspect of the question the method 

 of recording movements with a myograph was devised, and used 

 according to the method described in connection with figure 60. 

 Studies were made with this method upon the action of hydro- 

 chloric acid in as great dilution as n/10,000; with the result that 

 degrees of concentration of the acid which are not adequate to 

 stimulate the reflex mechanism were found to have a destructive 

 action upon the skin perceptible under the microscope. 



In figure 60, graphs A, B and C represent respectively the 

 action of HCl n/300, n/400 and n/500 upon the behavior of 

 embryos of Rana catesbiana of the advanced swimming stage. 

 The solid line of each graph represents the composite of the 

 ' activity of five specimens in the acid, while the broken litie 

 represents the composite of the activity of the same five speci- 

 mens in pond water following the same kind of mechanical 

 agitation in changing from dish to dish as occurred in the manip- 

 ulation with acid. The reaction in water was in each case taken 

 immediately before the reaction in acid. 



In the three graphs on the action of acid there is a striking 

 similarity; but the most noteworthy result of this method of 

 study is that the composite of the activity of a number of speci- 

 mens can be represented by a curve. Particularly is this impres- 

 sive when one knows that, although these very embryos, were re- 

 moved from the acid immediately following the experiment 

 (after an immersion of less than seventy-five seconds) they 

 showed unmistakable evidence of the destructive action of the 

 stimulating agent upon the skin. In addition, these graphs 

 seem to show, particularly when the increasing normal activity 

 as indicated by the broken line is taken into account, that the 

 intensity of the response (the height of the curve) increases, while 

 the duration of the response (the length of the curve) decreases 



