THE PERMEABILITY OF FROG SKIX TO UREA. 245 



higher concentration of CaCl 2 in the external solution to produce 

 a concentration of about 10 per cent, in the region of the greater 

 number of the cells. In support of this idea the figures presented 

 show that sometimes the minimum amount of urea penetration 

 was obtained with 25 per cent, or 50 per cent. CaClo, or even in 

 one case in pure isotonic CaClo solution, as in Table IV., quoted. 

 \Yhen the urea was dissolved in the same mixture of isotonic 

 salt solution as that used on the other side of the skin, the 

 concentration of CaClo within the inner strata of the skin was 

 probably much more easily reached, and the most striking 

 differences in the permeability to urea was shown with experi- 

 ments thus carried out. 



SUMMARY AND CONCLUSIONS. 



Experiments were carried out on the effects of NaCl and 

 CaClo alone and in combination upon the permeability of frog 

 skin to urea, with the following results. 



Frog skins bathed in isotonic solutions containing varying 

 proportions of CaC^ and NaCl showed less permeability to 

 urea than did the controls of matched skins bathed in pure 

 isotonic NaCl. 



The minimum amount of penetration of urea, while varying 

 much with different skins, was obtained on an average with 

 about 10 per cent, of a i.i per cent. CaClo solution plus 90 

 per cent, of a 0.7 per cent. NaCl solution. 



In many cases, isotonic CaClo solutions alone as compared 

 with pure NaCl solutions increased the permeability to urea. 



The writer is indebted to Dr. M. H. Jacobs for suggesting the 

 study of urea with frog skin and for his continued interest and 

 criticism. 



This work was first undertaken at the University of Pennsyl- 

 vania during the year of 1922-23, but could not be completed 

 until facilities were obtainable again at the University of Texas 

 during 1925-26. This accounts for the long break in the date 

 of the experiments. 



