168 Regulation of Size in Unicellular Organisms 



nificant changes of bodily proportions accompanied 

 the change of viscosity. The viscosity of Stentor was 

 found by Heilbrunn ('23) to be markedly modified in 

 either direction by the presence of certain cations in 

 the medium. 



In contrast to the high viscosity of these ciliates 

 stands the value for the internal mass or plasmasol of 

 Ameba. Heilbrunn ('29) ascertained that this was 

 only twice the viscosity of water. 



In Ameba there is a continuous local change of vis- 

 cosity of the body substance; in this sort of organism 

 it is doubtful whether size could be a simple correlative 

 of the ratio of surface forces to internal forces. Chalk- 

 ley ('29) made some measurements of relative volumes 

 of fluid and solid portions under various conditions; 

 but it seems probable that some ratio which would af- 

 fect size would be much less than the errors of his 

 measurements. 



Surface tension and fission. One of the favorite 

 "theories" of cell division has been that the lowering 

 of surface tension relative to massive forces results in 

 fission. Perhaps such a hypothesis is only a descrip- 

 tion which adds nothing to direct observation. But 

 the hypothesis leads to a definite type of experiment; 

 namely, actually to lower the surface tension between 

 medium and organism to see if smaller body sizes then 

 prevail. Up to the present time, however, the only ex- 

 periments of this sort have not measured body size, 

 but rather shape, rate of multiplication, flotation, in- 

 fection, and viability (Larson, '28). The rate of multi- 

 plication of some species of bacteria was found to be 

 decreased by the addition of soaps, which can equally 

 readily be explained as an effect of some other sort 

 than surface tension. Indeed, to avoid the surface 

 effect of the reagent, the body may easily contain a 

 concentration of the substance proportional to that of 



