256 M. L. BASKERVILL. 



DISCUSSION. 



The experiments show that in the case of the shorter exposures 

 the passage of urea through frog skin was checked by the presence 

 of sugar. After about six hours, the time differing with different 

 skins, the skins in a pure sugar solution were greatly injured, 

 especially if the sugar was in contact with their inner surfaces. 

 As a rule, skins in mixtures of sugar and salt solutions survived 

 in proportion to the amount of salt present. In contrast to 

 these conditions, skins bathed on the inner surface by Ringer's 

 solution survived 24 hours and upward. Accordingly, when 

 skins were exposed to the solutions for six hours or more, the 

 skins in contact with the solutions containing the larger pro- 

 portions of sugar were the more injured and permitted more 

 urea to pass. Skins that had been killed in various ways showed 

 more nearly the same degree of permeability in Ringer's and in 

 sugar solutions. 



As in all work on permeability of cells and tissues, it is necessary 

 to distinguish carefully between the permeability caused by 

 injury and that normal to tissues. It is not claimed that in 

 these experiments the skins remained entirely uninjured during 

 the time three or four hours that constituted the minimum 

 exposure to the solutions used, which are quite abnormal for 

 frog skin; but it is believed that the skins remained in a 

 sufficiently healthy condition for this length of time to exercise a 

 considerable degree of the selective permeability that is charac- 

 teristic of living tissues. In spite of the fact that skins exposed 

 to the sugar solutions even for the shorter periods would be 

 expected to be proportionately more injured than those exposed 

 to Ringer's solutions, more permeability to urea was shown in 

 the latter condition. 



Because of these results on urea it may be concluded, therefore, 

 that the favorable effects of salts on the penetration of dissolved 

 substances is not limited to the passage of other electrolytes as 

 some authors have assumed, but applies to certain non-electro- 

 lytes as well. This is not surprising considering the universal 

 presence and the known physiological importance of the electro- 

 lytes in living cells and tissues. 



