Membrane structure as revealed by permeability studies 



which was also taken by Capraro et al. (1952) who obtained similar results with 

 isolated frog skins. There was, however, something mysterious about this apparant 

 active transport. It could take place only if there was an osmotic gradient to help it. 

 With isotonic sucrose in the outside compartment and Ringer solution inside, the 

 net water transfer was nil. This is in keeping with the observation made by Krogh 

 years ago that frogs placed in isotonic sucrose will not take up water and do not form 



Table I 



Effect of neurohypophysial hormone on influx and net flux of water through toad skin. Inside 

 solution, Ringer solution; outside solution, 1/10 Ringer (Koefoed-Johnsen and Ussing, 1952) 



M in = influx of water (jul./cm. 2 /hr.) 



A w = net flux of water (/zl./cm. 2 /hr.) 



any urine. It is true that the isolated skin with Ringer solution on both sides performs 

 a transfer of water from the outside medium to that inside (Huf, 1936), but this is 

 probably connected with the active transport of sodium ions through the skin. 



The fact that the apparently active water transport needs an osmotic gradient to 

 help it started us wondering whether or not our basic concepts of the nature of os- 

 motic water transfer were correct. Does the net water transfer indeed arise as the 

 difference between the amount of water diffusing in and that diffusing out ? At first 

 sight even the asking of the question seemed to us preposterous, but on second 

 thoughts we realized that the problem needed reconsideration. 



35 



