THE PERMEABILITY OF FROG SKIX TO t'KKA. 249 



The permeability of frog skin to urea as influenced by electro- 

 lytes and by non-electrolytes may readily be ascertained, since 

 direct chemical determinations of the amount of the substance 

 passing through the skin may be made. Urea has the further 

 advantage of penetrating so readily that the tissues may maintain 

 their vitality through the period of experimentation. Hence it 

 seemed of interest to find how much urea will pass through frog 

 skin when it is bathed in Ringer's solution and when it is bathed 

 in sugar solutions. 



In experiments carried out during the winter of 1922-23 much 

 less urea penetrated into sucrose solutions than into Ringer's 

 solution. As soon as facilities for the work could be secured 

 again, the experiments were repeated and extended to dextrose. 

 While the results quoted are in most cases the more recent ones, 

 they do not differ in kind from those obtained several years 

 previously. 



MATERIALS AND METHODS. 



The procedure was similar to that described in the fore- 

 going paper (10). Matched pairs of frog skin were put over 

 1 6 mm. tubes. Into one tube of each pair was measured a 

 convenient amount, usually 2.5 cc. of the sugar solution; into 

 the other, the same amount of Ringer's solution. Each tube 

 was then placed with its other surface bathed by the urea solution. 

 All solutions referred to were isotonic except the urea solution, 

 which was five per cent. In preparation for the experiment, 

 each skin was usually washed about an hour in the particular 

 solution that was to be used on the inside of the tube. The 

 exposure to urea was continued for varying times. Series of 

 experiments were carried out with both dextrose and sucrose. 



\Yhen the study of the permeability of frog skin to urea was 

 begun, much time was spent in trying to discover if the orientation 

 of the skin made any difference in its permeability to urea. 

 Przylecki (n) has reported that urea penetrates frog skin equally 

 well in both directions. Under the conditions employed in this 

 paper it was found to penetrate readily in either direction, while 

 no marked difference in the rate of penetration in the two 

 directions was shown except in tubes where a solution used 

 might be more injurious to one side of the skin than to the 



