THE PERMEABILITY OF FROG SKIX TO UREA. 24! 



large enough to prevent any significant change of concentration 

 during the experiment. 



The skins were used in symmetrical pairs; so that with each 

 pair of skins taken from corresponding parts of the two sides of 

 the frog, one was treated with NaCl (0.7 per cent.), and the 

 other with isotonic CaCl2 (i.i per cent.), used either alone or in 

 combination with the NaCl solutions. All references to salts in 

 this paper are to solutions of the above mentioned concentrations. 



After a convenient interval of time, varying usually from 

 two to seven hours, the amount of urea that had penetrated 

 the skin was measured by the Van Slyke-Cullen method (8). 

 Numerous experiments were performed because conclusive re- 

 sults could be obtained only by frequent repetition of each 

 experiment in order to compensate for the high degree of vari- 

 ability in the skin, in part seasonal, in part individual. With 

 frogs that had been in captivity for some months, and especially 

 with frogs that had developed "red-leg," no significant difference 

 in the permeability could be obtained. However, even in these 

 skins such slight differences as were noted were virtually always 

 in the same direction as in normal skin. 



In most of the experiments a 5 per cent, solution of urea was 

 used, as lower concentrations did not yield significant differences. 

 In order to subject, as far as possible, all of the cells of the skin 

 to the desired proportions of salts, the urea was usually dissolved 

 in the same isotonic salt mixtures as were used on the other 

 side of the skin. Although the objection might be raised that 

 this strongly hypertonic solution penetrates readily by reason of 

 injurying the tissues, there is evidence to the contrary. Skins 

 that have been killed allow equal quantities of urea to pass 

 whether NaCl or NaCl and CaClo is used; the fresh skins, 

 however, show marked differences of behavior under the con- 

 ditions of the experiments. Furthermore, if an isotonic solution 

 of urea, i.e., 1.5 per cent., is used with one of each pair of skins 

 in a series, and the other skin is treated with 1.5 per cent, urea 

 solution brought up by means of glucose to the same osmotic 

 pressure as a 5 per cent, urea solution, there is no essential 

 difference in the passage of urea in the two series. For these 

 reasons it is felt that the experiments reported are no more 



