THE NERVOUS SYSTEM OF AMPHIBIA 303 



2. Since, according to Crozier the "concentrations of irritants 

 employed b}^ Parker and others" "do not penetrate at all" are 

 there free nerve endings at the surface of the skin of vertebrates 

 to function as receptors for these chemical stimuli (see Herrick- 

 Coghill, '98)? 



3. Since theepidermis of fishes and amphibians may be thought 

 of as a system of three colloidal agglomerations, normally in 

 equilibrium with each other — (1) the cellular protoplasm, and (2) 

 surface films of mucus-like substance, which bathe opposite 

 surfaces of (3) the cell membrane — Jiiay not very great dis- 

 turbances of equilibrium with consequent violent effect upon 

 neighboring parts result from the addition of a foreign solvent 

 or solute to the surface film actually without the added solvent 

 or solute penetrating the cell membrane (Nelson, '13, particu- 

 larly table 5)? 



4. In order to stinuilate the tactile nerve endings mechanically 

 through the reaction of the epithelial cells nmst the action of 

 the reagent be so violent as to produce histologically or chemically 

 demonstrable effect upon these cells? May not relatively slight 

 increase or decrease in the turgor of the epithelial cells be suffi- 

 cient to stimulate? 



5. Why should the stinuilating effect of water when ap- 

 plied after the application of a chemical stinnilus be evidence 

 of destruction of tissue in one case ("If any serious disintegration 

 were produced by these solutions, it would be reasonable to expect 

 the continuance of activity after external supply of the stimu- 

 lant had been removed." Crozier, '16, p. 3) and in another case 

 be evidence of a 'general chemical sense' ("Washing the foot 

 with distilled water does not lead to cessation of the contraction, 

 because, after exposure of the foot to certain solutions (Loeb 

 '05), water stimulates" Crozier, '16, p. 5)? 



6. How can this last mentioned irritability to water after 

 chemical stimulation be explained upon the hypothesis of a "gen- 

 eral chemical sense," and if it can be so explained why should it 

 appear so conspicuoush^ in the behavior of amphibian embryos 

 (Coghill, '14. ])p. 197, 198) the responses of which, Crozier con- 



