60 DAVID R. BRIGGS 



least change in volume of the specimen, the osmotic pressure of the 

 solution contained within the cells can be obtained. Solutions of 

 osmotic pressure greater than that of the specimen contents (hy- 

 pertonic solutions) will cause shrinkage (plasmolysis) of the cells 

 due to passage of water out of the cell interior while solutions of os- 

 motic pressure less than that of the specimen (hypotonic solution) will 

 allow swelling of the cells. That solution with the same osmotic 

 pressure as the cell contents (isotonic solution) will cause no change 

 in its volume. It is essential to bear in mind that the validity of the 

 results of such determinations will depend upon the efficiency with 

 which the cell membrane acts in a semipermeable manner both to the 

 solutes contained in the cell and to the solute components of the test 

 solution. This is usually a point of considerable uncertainty. If the 

 membrane were permeable to solvent but completely impermeable 

 to all solutes both within and outside the cell, the relative activity of 

 the solvent on the two sides of the membrane would be the sole de- 

 termining factor for the passage of solvent. On the other hand if 

 the membrane were completely permeable to all molecular species 

 both solutes and solvent, and if the diffusion constants of the solute 

 components in both solutions were of about the same magnitude, 

 again little or no net transfer of solvent would occur when the two 

 solutions were isotonic. If, however, the membrane possesses a dif- 

 ferential permeability to various solute components or if the diffusion 

 constants of various solute components were very different, some 

 transfer of solvent would occur even though the two solutions bathing 

 the opposite sides of the membrane were initially isotonic. It can 

 generally be assumed that water will move across the membrane with 

 greater ease than will any of the solutes, so that, if the observations 

 are based on the initial trend, more reliance might theoretically be 

 placed upon them than when the observations are made at equilib- 

 rium. A disturbing circumstance exists, when initial changes in 

 volume are being employed in such determinations, in that membrane 

 potentials (electrical) that may occur across the membrane can, 

 through electroosmotic effects, constitute the most important factor 

 determining the initial passage of water across the membrane (21). 

 Anomalous osmosis resulting from the performance of work by ex- 

 penditure of electrical energy in such cases would destroy the vahdity 

 of these results when interpreted as due to osmotic forces alone. 

 Many cell membranes, however, appear to act as truly semiperme- 

 able when the solutes of the test solution are not greatly divergent 



