62 DIFFUSION OF SUBSTANCES INTO LIVING CELLS 



determined, but we do know the more important laws 

 which are sufficient for practical purposes. 



The diffusion of substances into living cells is purely 

 a physical process. A cell does not seem able to exert 

 any vital control or power of selection whatever over 

 the diffusion of substances into its cytoplasm. In every 

 cell with which we have experimented this has been the 

 case, and if proof is needed it is afforded by the fact 

 that we can at will cause cells to be excited, to repro- 

 duce themselves, or to die, by employing the knowledge 

 of the laws, which shall presently be described, enabling 

 us to make substances diffuse into cells at any speed we 

 please. 



Owing to a variety of means at our disposal cer- 

 tain cells can be made to die in two minutes or in 

 two hours, whichever one likes, merely by accelerating 

 or delaying the diffusion of the agents into them; and 

 it is clear that if a cell could control this diffusion it 

 would at least make some effort to do so in order to 

 save its own life. They cannot do so, however, and 

 always die with clockwork-like regularity at the end of 

 various given periods of time, which are determined by 

 controlling the diffusion of certain chemical substances 

 into the cell's cytoplasm. As will be shown later, the 

 rate of diffusion of substances into living cells can be 

 calculated by means of a simple equation; and since 

 excitation, reproduction, and death can each in succes- 

 sion be induced in vitro by causing the diffusion of 

 substances into cells, it follows that excitation, repro- 

 duction, and death may also be induced according to 

 the rules which can be plotted as a simple equation. 



