ERNST G. HUF 211 



One may think in terms of increased friction or decreased effective pore size. 

 This would not apply, as one can easily see, for an osmotic water flow in the 

 outward direction, since this is also the direction of the slowly moving water 

 tilm, according to our assumptions. 



One may suggest, perhaps, that one-way osmosis in surviving frog skin be 

 interpreted as the result of anomalous osmosis superimposed on normal osmosis. 

 Anomalous osmosis is an electrokinetic phenomenon (66). Such an interpreta- 

 tion is obviously at hand. It is well known that under most circumstances the 

 inside of the skin is electrically positive with relation to the outside. Since the 

 skin potential, however, is just as dependent on the metabolism of the skin 

 (4-6, 15) as is active salt transport (16, 17), it seems to become a matter of 

 definition as to whether one should speak of active water transport or of anom- 

 alous osmosis in surviving frog skin. It would be interesting, nevertheless, to 

 look for a closer correlation between 'one-way osmosis' in surviving skin and 

 the potential differences across skin, while varying, over a wide range, the 

 concentration gradients of salt solutions across the skin. Some background for 

 this has already been established by Steinbach (67; fig. 4). It can be seen from 

 the graph that the outside, much more than the inside, of skin is sensitive to 

 changes in total concentration of salt. In the case of 'one-way osmosis' which 

 has been presented in figure 3, one must conclude, from Steinbach's potential 

 measurements, that a five times diluted Ringer's at the outside, as compared 

 to normal Ringer's, did not result in a significant increase in total potential. 



ACTIVE UPTAKE OF SODIUM CHLORIDE 



Soon after it was well established that active fluid transport takes place in 

 surviving skin, it was also found that the transported saline solution was 

 hypertonic with respect to the saline solutions on either side of the skin (15, 17). 

 In other words, salt (NaCl) accumulated at the corium side of the skin. This 

 observation has led to a great number of studies from various laboratories. 

 One aim was to search for essential chemical and physico-chemical factors for 

 the maintenance of active salt transport. Other studies were made with the 

 primary aim of arriving, at a suggestive level at least, at some picture of the 

 mechanism of salt accumulation in frog skin. 



That the accumulation of strong electrolytes in living cells presents a problem 

 of peculiar interest was clearly recognized and stated in 1926 by W. J. V. 

 Osterhout (53). He has made significant contributions to this field. Fortunately, 

 there are many workers now directing their efforts towards the same goal that 

 has fascinated Osterhout for nearly 30 years (55). Every student of general 

 physiology, at one time or another, will hear about the 'carrier hypothesis' of 

 active ion transport, no matter whether it involves anions or cations, or whether 

 it is in connection with V'alonia or other cells and with tissues, such as frog 

 skin. The 'carrier hypothesis' presents itself in greatly retined concepts today 



