It is generally recognized that the permeability of cells is markedly affected by temperature ; and numerical temperature coefficients have been determined in a number of cases to measure the magnitude of this effect (for examples see the bibliographies given by Hober, '26, chapter 8; Stiles, '24; Jacobs, '24; and Gellhorn, '29). Some of the data in the literature are of very questionable value (for criticisms see Delf, '16, arid Jacobs, 'ZS), and in most cases, even when otherwise entirely satisfactory, are of limited significance, throwing light upon the effects of temperature changes on the permeability of single kinds of cells to single substances, but not, as a rule, permitting quantitative comparisons of the effects of such changes upon difffforent cells. For such comparisons to be made, it is necessary that permeability be measured in more precise terms, sucli as, for example, the amount of a given substance that will enter a given cell through unit area in unit time with unit concentration gradient, or, when it is impossible or unnecessary to separate the factor of its thickness from the other factors determining the permeability of the cell membrane, with unit difference of concentration or of osmotic pressure bet-i-veen the cell and its surroundings.