THE NERVOUS SYSTEM OF AMPHIBIA 205 



by Hooker ('11) is of interest. Hooker's experiments were made 

 to test the property of the skin as a conductor of impulses lon- 

 gitudinally in the trunk. Incidentally they lend credence to the 

 idea, if they do not certainly demonstrate, that the subepithelial 

 terminals of the giant ganglion cells do not form a conducting 

 syncytium in or beneath the skin. This is in harmony with my 

 experiments in which severing of the dorsal portion of the cord 

 made it impossible to stimulate response to a light touch applied 

 caudad of the lesion. There is obviously here no cutaneous or 

 subcutaneous structure which can conduct stimuli longitudinally 

 in the trunk for any considerable distance. 



My experience with hydrochloric acid as a stimulating agent 

 have important bearings upon the work of Parker ('12), Sheldon 

 ('09), Cole ('10) and others upon fishes and Amphibia with refer- 

 ence to a general or ''common chemical sense " in the skin. These 

 authors hold the view that there is a special set of receptors which 

 are normally irritable to various substances in solution. My 

 conclusions do not harmonize with this view so far as amphibian 

 embryos are concerned. Moreover, a comparison of my results 

 in detail with those of Parker, Sheldon and Cole convinces me 

 that the burden of proof is still upon them as regards the nervous 

 irritability of the skin to chemical stimuli in fishes and amphib- 

 ians generally; for every essential characteristic of response which 

 they describe can be duplicated in amphibian embryos which are 

 known to be responding to a violent disruptive action of the 

 stimulating chemical agent upon the epithelium of the skin. 



To understand how the response of fishes and amphibians to 

 acid, or other substances in solution, may be caused by the de- 

 structive action of the stimulating agent, it is only necessary 

 to recall that the deeper cells of the cutaneous epithelium of all 

 vertebrates are permanently embryonic; and that the general 

 cutaneous nerve fibers end among these embryonic cells as the 

 terminals of giant ganghon cells end upon the deeper layer of 

 epithelial cells of the amphibian embryo. My experiments upon 

 young, ciliated embryos with HCl n/1000 and n/10,000 show that 

 the cutaneous epithelial cells of the embryo, regardless of nerve 

 endings, are beyond comparison with the skin of fishes in sen- 



