INTRINSIC ACTIVITY OF SECRETING CELLS 147 



or in such, very different and fixed concentrations within and 

 without the cell. For example, that potassium salts are found 

 in the cell in excess, and sodium salts in the plasma in excess, and 

 that this is due to a membrane refusing passage entirely to sodium 

 and potassium ions. Then this excludes all exchange of such 

 ions between cells and plasma, and there is neither any explana- 

 tion on such a basis of how the present state of affairs with such 

 an unequal distribution of potassium and sodium ions occurred 

 in the first instance when the cells were formed and growing ; 

 nor any explanation of how more potassium ion is taken in and 

 sodium 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 level in potassium and sodium salts. 

 But there is no explanation whatever of how that equilibrium 

 was attained initially, nor how it is maintained when the 

 cell volume increases as cell multiplication occurs. Are we to sup- 

 pose 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- 



1 See footnote on p. 145, and also p. 164. 



