206 G. E. COGHILL 



sitiveness to the action of acid. They would, in fact, explode 

 almost instantaneously upon immersion in such concentrations of 

 acid as were used by Parker and Sheldon in their experiments 

 upon fishes. It is my opinion that the deeper embryonic cells 

 of the skin of adult fishes and amphibians would act in the same 

 way if exposed directly to the acid. In their normal condition, 

 however, they are bound down and protected by a thick, less 

 sensitive and more impervious layer of cells. Under such con- 

 dition the acid must cause exceedingly violent mechanical dis- 

 turbances beneath a comparatively passive exterior; while the 

 less sensitive outer layer of cells protect the deeper cells during 

 processes of repair when the destructive action ceases. 



Sheldon's experiments in which he found that areas of the skin 

 which have become fatigued to tactile stimulation are still sensi- 

 tive to various chemical agents, and that areas that have been 

 stimulated by chemical agents are thereafter for a time insen- 

 sitive to tactile stimulation, are exactly in line with my obser- 

 vation of embryos immersed in HCl n/2000, n/3000 and n/4000. 

 His experiments mean to me that the chemical agent has not 

 fatigued but destroyed the sensitive portions of the skin to such 

 an extent as to render it inert to normal stimulation; while the 

 "fatigue" to tactile stimulation is merely adaptation, just as my 

 ear is now so adapted to the sound of the clock that the tick 

 does not affect my motor system. These experiments of Shel- 

 don's, to my mind, illustrate exactly the difference between the 

 effect of normal stimulation and destructive action by stimulat- 

 ing agents. 



In like manner, Cole's experiments with cocaine upon frogs 

 which were stimulated with chlorides are not conclusive on the 

 point for which the author is contending. In interpreting this 

 experiment it must be considered that the action of the chloride 

 is much more extensive and affects many more cells at once than 

 does any of the single acts of pinching or pricking which Cole 

 employed to test the effect of the cocainization upon the receptors 

 to tactile stimuli. Furthermore chemical action has a much 

 more violent and destructive effect upon protoplasm than does 

 mechanical stress and strain, as may be experienced, for instance, 



