SALT AND WATER IN LIFE AND GROWTH 113 



a semi-permeable membrane. Owing to this semi-per- 

 meability the gelatin drop swells if salt or sugar is added to 

 it (Fig. 32b). The salt inside attracts water and draws it 

 through the membrane, since the salt itself cannot pass 

 through the film. But if salt or sugar is added to the tannic 

 acid on the outside, the drop will shrivel, because now the 

 salt draws water from the inside of the drop through the 

 membrane (Fig. 32c). We see, therefore, that the flow of 

 water through the semi-permeable membrane occurs towards 

 the dissolved substance. These results can also be expressed 

 in a different way. We may say the swelling is due to a 

 pressure exerted by the salt solution, if this is inside. Such 

 a cell may be compared to a rubber balloon which is puffed 

 up by blowing air into it. But this analogy is incomplete 

 since the rubber balloon which is puffed up certainly does 

 not gain weight; as it is stretched it gets thinner. In this 

 regard the artificial cell behaves differently; the iridescent 

 film on the gelatin in tannic acid solution does not get 

 thinner. More of it is being formed, owing to the expan- 

 sion of gelatin which offers a greater surface to the tannic 

 acid solution, allowing this solution to act upon a greater 

 quantity of the gelatin. Hence the film actually grows by 

 stretching. If similar conditions prevail in living tissue, 

 we should expect that living cells will also grow under the 

 expanding influence of certain substances in the fluids inside 

 the cells. 



4. THE SWELLING AND SHRINKING OF LIVING CELLS 



Doubt may arise whether there is true similarity between 

 a living cell and the artificial cell made of gelatin and tannic 

 acid. We need definite evidence that similar conditions 

 occur in living tissue. Does a living cell shrivel or swell 

 under the influence of salt in the same fashion as our arti- 

 ficial cell? This question is readily answered by observing 

 red blood cells. The blood cells are actually tiny bags 



