v ELECTROMOTIVE ACTION OF EPITHELIAL AND GLAND CELLS 479 



skin of the fish, or, more correctly, every spot on the superficies 

 of the fish, is, in exactly the same way as the skin of the frog, 

 the seat of an electromotive force directed from without inwards, 

 and very easily disturbed by corrosion." In the eel it is not 

 difficult to strip off the entire skin, or to prepare pieces of it, 

 but it is advisable in all cases that the fish under examination 

 should be as fresh and uninjured as possible, since the electro- 

 motive activity easily receives a permanent check from any slight 

 injury. E. Waymouth Keid and Tolputt (83) have recently 

 observed reversal of the current on fatigued animals. 



Under normal conditions the rest current of the frog's skin 

 agrees perfectly with that of the fish save from its greater 

 strength in most cases although the histological structure of the 

 two objects presents fundamental differences. Mucous cells are 

 not, as with the fish, the chief constituents of the surface 

 epithelium proper, but are confined almost exclusively to the 

 multicellular skin glands ; the epithelium, on the other hand, is 

 composed almost exclusively of polyangular prickle and bristle 

 cells, those next to the cutis being more cylindrical in shape, 

 while towards the surface they get more and more flattened, and 

 are eventually covered over with a single layer of flattened epi- 

 thelium. Only a few solitary goblet cells, small and flask-shaped, 

 are found in the epithelium, near the surface, and even these 

 (according to F. E. Schultze) do not open upon it. 



Engelmann's observations (as confirmed by the author) 

 are the best authority in regard to the normal entering rest 

 current of the frog's skin which is mainly to be referred to 

 the great number of skin glands present. 



The dependence of E.M.F. in the skin current upon the 

 bulk of water in the tissues is once more apparent. We 

 can readily see that the current will become weaker, in pro- 

 portion as the epidermis gets drier, since the resistance to con- 

 ductivity increases enormously with the latter. Simple moistening 

 with water or dilute salt solution produces a rapid and consider- 

 able increase of E.M.F. in each such case. The greatest remainder 

 of E.M.F. is obtained with pure water. " If a drop of salt solu- 

 tion of 0-2 % is applied, after washing with water has 

 brought the E.M.F. to a constant height, it begins to decrease 

 after a few minutes. Eepeated dropping of the same solution 

 depresses it still further, until it reaches a constant level. A 



