136 



W. J. V. OSTERHOUT 



has a further beneficial action after the penetration has been 

 accompUshed. The writer beheves that CaCl2 has a favorable 

 influence in both these ways. Evidence for this will be presented 

 later. It may be added that the beneficial effect of the CaCL 

 is so great that the appearance of the cell shows none of the char- 

 acteristic disturbances which NaCl alone produces, even in a 

 dilute solution. 



It may be asked how merely delaying the entrance of a salt 

 can protect the protoplasm against its toxic action. In this con- 

 nection we may recall the familiar phenomenon of colloid chem- 

 istry that a salt which produces marked effects when added sud- 

 denl}^ may produce little or no effect when added slowly. 



The results of these experiments may be briefly summarized 

 as follows: 



In the first place they show that Overton's hypothesis is un- 

 tenable, since it requires that only those salts which are soluble 

 in lipoid should be able to penetrate. 



In the second place they prove the correctness of Loeb's sug- 

 gestion that the antagonistic action of one salt on another is due 

 to the fact that one salt hinders the other from entering the cell. 



Thirdly, they indicate that the plasma membrane is protein 

 rather than lipoid. 



Fourthly, they show that the usual methods of determining 

 the osmotic pressure of the cell by means of salts are faulty. 



Finally they make it plain that true and false plasmolysis are 

 often confused in a way that may lead to serious error. 



These conclusions have all been very carefully tested by an 

 entirely different method which permits great accuracy, namely 

 by measuring the electrical resistance of the living tissue. 



The electrical method depends on the fact that the electric 

 charges in a solution are carried solely by ions, and that anything 

 which impedes the passage of ions will proportionally increase 

 the electrical resistance. The method, therefore, consists in 

 passing a weak current of electricity through the living tissues 

 and measuring their resistance by means of a Wheatstone bridge. 



To obtain reliable results in conductivity experifnents material 

 should be used which is not injured by weak currents or by other 



