254 INTRINSIC ACTIVITY OF SECRETING CELLS 



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 main- 

 tained, 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, should the basis of explanation be changed, and 

 when the cell comes into a position of labile equilibrium with its 

 media should it be supposed that, instead of those forms of 

 energy which brought it to that state being still active, 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 mem- 

 branes of an impermeable type are formed around the molecules 

 which keep them from reacting. No, the reaction is preserved by 

 the balance of opposing factors, reactions still occur between the 

 molecules, but these are equal and opposite. 



So also in the case of the living cell in equilibrium: the case 

 is not that of an impenetrable membrane through which an ion 

 of potassium or sodium never passes, but a labile equilibrium 

 with both potassium and sodium ions passing in and out all the 

 time, but the numbers passing in and out are equal, so that the 

 concentrations are preserved unaltered. 



That this is the true state of affairs there is abundant experi- 

 mental proof. For let the resting cell divide, and the two daughter 

 cells commence to grow, then the supposed impermeable membrane 

 for potassium ions quits the scene of action, and the growing cell 

 readily takes up potassium ions. 



