THE PERMEABILITY OF FROG SKIN TO UREA. 257 



SUMMARY. 



1. Isotonic dextrose or sucrose solutions, as compared with 

 Ringer's solution, check the passage of urea through frog skins 

 if the exposure to the solutions is not so prolonged that the skins 

 are injured. 



2. When the skins are kept in contact with the solutions for 

 such a length of time that those in the sugar solutions are injured 

 or killed and those in Ringer's solution are still in a healthy 

 state, then more urea may penetrate in the presence of the 

 sugar solution. 



3. Mixtures of isotonic sugar solution and Ringer's solution 

 check the penetration of urea in proportion to the amount of 

 sugar present when that amount reaches a concentration of 50 

 per cent, or more. 



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