2o6 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



regulation in lower animals. In this, of course, the body surface was largely 

 involved. My particular interest in the permeability properties of body surfaces 

 of aquatic animals was aroused by experiments on isolated skin of frogs de- 

 scribed by Reid in 1890 (58, 59), Maxwell in 1913 (49), and Wertheimer in 

 1923 (78). 



ACTIVE FLUID TRANSPORT AND ONE-WAY OSMOSIS 



The last three workers had found that a movement of fluid occurs from the 

 outside to the inside of isolated skin under conditions in which there exists no 

 hydrostatic or osmotic pressure difference across the skin. This, of course, was 

 not to be expected. The net rate of fluid transport is of the order of 100 to 

 200 /xM cm~- hr.~\ as we have now learned from careful studies (28). At this 

 rate, it would take about 300 to 600 hours for i ml of fluid to move across a 

 piece of skin of the size of i cm^. Because of this slow rate and for other reasons, 

 it was often felt that this kind of fluid transport might have been the result of 



Table i. Active fluid transport across frog skin 



Average data. 21 normal, 6 poisoned skins, with Ringer's on both sides. 



experimental errors and would not deserve any particular attention. The 

 situation changed, however, when it was shown (15-18, 20, 21) that suppression 

 of respiration of the skin causes fluid transport to become negligible (table i). 



A field closely related to 'active transport' is that of 'one-way exchange 

 processes'. This means that, in what appears to be a passive exchange of certain 

 substances across a membrane, the rate of exchange in one direction is greater 

 than the rate of exchange in the opposite direction. A variety of observations 

 could be classified under the heading of 'one-way exchange processes'. Active 

 transport could be regarded as one particular form of a 'one-way exchange 

 process', which, at the present time, occupies the minds of a great number of 

 biochemists and biophysicists. 



The earlier workers were quite fascinated by these 'one-way exchange 

 processes'. Because of the possible close relationship to active ion transport in 

 frog skin, I would like to devote some of my time to a discussion of what has 

 been called 'one-way osmosis' (19). I shall speak of inward osmosis, where 

 water flows from the epithelium towards the corium, and of outward osmosis. 



