THE NERVOUS SYSTEM OF AMPHIBIA 193 



Preliminary experiments showed that response could be ehcited 

 by the use of the various substances to which the dogfish reacts 

 in Sheldon's experiments, and that the action of the various in- 

 organic acids is essentially the same so far as response is concerned. 

 To simplify the experimentation, therefore, hydrochloric acid 

 was adopted as a type of chemical stimuli. According to Shel- 

 don's procedure a normal solution of the acid was prepared by 

 titration against a gram-molecule solution of an alkali. From 

 this stock of normal hydrochloric acid the various dilutions used 

 as stimulating agents were made. 



.My first experiments consisted in spraying the solution against 

 the embryo. As a means of applying such a stimulus with pre- 

 cision, pipettes were made from pieces of glass tubing, upon one 

 end of which a very thin-walled bulb was blown while the other 

 end was drawn out into a very fine capillary aperture. With the 

 acid enough methylene blue was mixed to give it a perceptible color, 

 so that the diffusion of the substance through the water could 

 be followed under the microscope (the Zeiss binocular being used 

 in these experiments). By this method a very fine jet of acid could 

 be applied to restricted areas of the skin as comparatively local- 

 ized stimuli. 



It was necessary, of course, in the application of this method, to 

 eliminate methylene blue and the mechanical impact of the spray 

 as factors in stimulation. This was done by spraying the em- 

 bryos with pure water and with a solution of methylene blue in 

 pure water. Such check experiments showed that the mechanical 

 impact of the spray and the methylene blue content of the solution 

 were negligible factors so far as stimulation is concerned. 



Embryos selected according to my typical physiological stages 

 as determined by reactions to tactile stimulation were tested 

 with various dilutions of the hydrochloric acid. Embryos of the 

 non-motile stage, as determined by tactile stimulation, gave no 

 response to the acid spray, while embryos that responded to tactile 

 stimulation responded also to the acid stimulation when sprayed 

 with dilutions as great as n/400. With stronger solutions the 

 reactions were prompt and vigorous, while with greater dilutions 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 24, NO. 2 



