INTRINSIC ACTIVITY OF SECRETING CELLS 253 



ion excluded when cell division takes place and new cells are growing 

 and causing increased volume of cellular tissue without any drop in 

 potassium ion concentration. The explanation on the ground of 

 complete impermeability can obviously only hold so long as the cell 

 is in complete equilibrium with the plasma as regards the inner and 

 outer levels in potassium and sodium salts. But there is no explana- 

 tion whatever of how that equilibrium was attained initially, nor 

 how it is maintained when the cell volume increases as cell multi- 

 plication occurs. Are we to suppose that the original fertilised 

 ovum contained all the potassium salts of the adult organism ? 

 Obviously such a conclusion is absurd, and it must be admitted that 

 the cells must have at some time taken up potassium and continued 

 to reject sodium ions. 



In fact, it is quite clear that so far from being impermeable 

 to potassium ions, up to the period at which the cell attained its 

 maximum saturation, it must have greedily taken up potassium 

 ions, from an exceedingly low concentration in the plasma, by 

 an active process of selective absorption against osmotic pressure 

 and with corresponding expenditure of energy by the cell, in the 

 same fashion as a diatom concentrates the silica for its skeleton 

 from the trace present in sea water, or as the bone-forming cells 

 take up the calcium salts from the circulating plasma. Once 

 the cell has attained its normal level of potassium ion concentra- 

 tion this action ceases and equilibrium is attained; but this con- 

 dition is preserved only so long as the cell is resting in size. There 

 is no evidence that there is an impermeable membrane formed, 

 or that the cell is really impermeable to potassium salts, because 

 it does not give them out or take them up any more in appreci- 

 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 appreciable extent; it 

 actively preserves itself against osmotic invasion. On the other 

 hand, when such a cell is placed in a solution not containing potas- 

 sium salts, such as a solution of sodium chloride, it does not part 

 with its potassium salts in appreciable amount ; but this need not be 



