148 INTRINSIC ACTIVITY OF SECRETING CELLS 



able quantity ; all this means is, that there is a balance being main- 

 tained dependent upon the nature and active properties of the 

 particular cell protoplasm involved. When such a cell is immersed 

 in a solution of a potassium salt it takes practically none up, 

 because it has already attained its balance in potassium ions, 

 and actively preserves this. Did it behave as an inert membrane, 

 as when it is killed, it would take up more potassium ions in a 

 strong solution ; but the living cell does not do so to any appreci- 

 able extent ; it actively preserves itself against osmotic invasion. 

 On the other hand, when such a cell is placed in a solution not 

 containing potassium salts, such as a solution of sodium chloride, 

 it does not part with its potassium salts in appreciable amount ; 

 but this need not be because it is surrounded by a membrane 

 impermeable to potassium ions but because it actively retains its 

 potassium ions on account of that affinity or activity by which 

 it originally took them up when they were present in traces only 

 in the plasma. 



Thus the balance of concentration for each individual ion 

 and salt and dissolved substance within and without the cell is 

 maintained, and readjusted when it changes, not by means of 

 any hypothetical inert impassable membrane stopping any reaction 

 between the cell contents and the constituents in solution within 

 and without, but by the play of the cell's activities upon the 

 medium in which it lives. 



This, it may be remarked, is not theory but experimental fact ; 

 we see that the growing cell takes up certain definite constituents 

 from the medium and rejects others, that the constituents taken 

 up are often taken up in opposition to osmotic pressure, and hence 

 only possible by the expenditure of energy by the cell. Why, 

 then, when the cell comes into a position of labile equilibrium with 

 its medium should the basis of explanation be changed, and it 

 be supposed that instead of those forms of energy which brought 

 the cell to that state, being still active in maintaining it in that 

 state, the mechanism of a hypothetical membrane or permeability 

 be invoked ? 



The condition is analogous to that of a chemical reaction which 

 has come into equilibrium ; here we do not suppose that the reaction 

 is frozen rigid, so to speak, at the equilibrium point, or that 

 membranes of an impermeable type are formed around the mole- 

 cules which keep them from reacting. No, the reaction is pre- 



