PHTSIOLOGT 15 



observing the formation of crystals of calcium 

 oxalate within the cell. It is evident, however, 

 that these methods have but limited application, 

 and that in many cases they are open to the ob- 

 jection that the penetrating substance injures the 

 cell. 



The penetration of a substance may some- 

 times be demonstrated by observing its effect 

 upon metabolism, but this method is inadequate 

 from a quantitative standpoint. Some investiga- 

 tors contend that substances may produce effects 

 on metabolism by their action at the surface, 

 without actually penetrating the cell. 



Owing to these difficulties a number of indi- 

 rect methods have been employed. Of these 

 plasmolysis has been the most popular. The 

 principle is very simple. If to the fluid surround- 

 ing the cell a substance be added in sufficient con- 

 centration it will cause the withdrawal of water 

 and shrinkage of the protoplasm, but if this sub- 

 stance subsequently penetrates, the osmotic pres- 

 sure inside will increase and water will be taken 

 up. When the concentration of the substance in- 

 side equals that outside, the cell will contain ap-V / 



