512 EDMUND NEWTON HARVEY 



to be noted especially between rest and activity, due to stimula- 

 tion. A further source of error results from the fact that the 

 very substance whose penetration we are studying may change 

 the permeability of the cell in the concentrations used. This 

 naturally leads to a brief consideration of the jnethods of test- 

 ing permeability used by various authors. They may be con- 

 veniently classified as follows: 



3. Metliods of testing permeahility 



1. Plasmolytic or osmometric (depending on changes in 

 volume and turgor of the cell). 



a. Direct observation of plasmolysis (DeVries, Overton). 



b. Indirect, by noting cessation of movement in motile bac- 

 teria (Wladimiroff, Zeit. f. physik. Chem. 7 p. 527, 1891). 



c. Indirect, by weighing (Overton, Pfliiger's Archiv. 92, p. 115, 

 1902, Loeb, Pfliiger's Archiv. 69, p. 1, 1897 and E. Cooke, Jour- 

 nal of Physiology, 23, p. 137, 1898). 



d. Indirect, by noting liberation of haemoglobin in isotonic 

 permeating solutions (Gryns, Pfliiger's Archiv. 63, p. 86, 1896). 



e. Indirect, by determining change in volume of centrifuged 

 corpuscles (Koeppe, Gryns, Hedin). 



/. Indirect, by determining the concentration in sea water and 

 the concentration, pure, capable of causing artificial partheno- 

 genesis (Loeb. Univ. of Calif. Pub. 3, p. 81, 1908) 



2. Observational a. Directly, as the entrance of dyes (Pfeffer, 

 Overton, Hoeber, Ruhland). 



b. Introduction of an indicator (as neutral red) and subse- 

 quent change. 



c. By some change produced in a substance (as tannin) already 

 present in the cell (Overton, Zeit. Physik. Chem. 22, p. 189, 1897). 



d. By microchemical tests (as the determination of KNO3 by 

 diphenylamin ; Molish). 



