PROTOPLASMIC STRUCTURE 113 



or dyes like neutral red' which may be introduced from 

 outside. These methods are especially valuable in 

 studying the permeability to acids or alkalis. Per- 

 meabihty to dyes may usually be studied by direct 

 observation. 



The method of plasmolysis, first employed systemati- 

 cally by Overton, is based on the production of osmotic 

 effects (entrance or exit of water) when the cell is placed 

 in solutions having a different osmotic pressure from 

 that of the cell-contents (anisotonic solutions). In 

 hypertonic solutions of substances which do not readily 

 traverse the plasma membrane the cell shrinks, in 

 hypotonic solutions it swells, while in isotonic solutions 

 its volume remains unchanged. The problem is to 

 determine the relative degree of permeability to differ- 

 ent substances, some of which may traverse the mem- 

 brane, but with unequal readiness. To do this the 

 behavior of the cell is observed in a hypertonic solution 

 of the substance under examination. It is clear that 

 when the dissolved substance in the external medium 

 is quite unable to penetrate the membrane, it exerts 

 pressure against the latter (by the continued impacts 

 of the molecules), and since this pressure is greater than 

 that exerted by the dissolved molecules within the cell, 

 water is extracted (or expressed) from the latter until a 

 permanent state of equilibrium is reached in which the 

 osmotic pressure of the cell-contents equals that of the 

 surrounding solution. The cell is then permanently 

 shrunken. When, however, the dissolved substance 



^Bethe, Arch. ges. Physiol., CXXVII (1909), 219; Warburg, 

 Z. physiol. Chem., LXVI (1910), 305; Harvey, Journal of Experimental 

 Zoology, X (191 1), 507; Chambers, Jour. Gen. Physiol., V (1922), 189. 



