154 Regulation of Size in Unicellular Organisms 



to be drawn from this comparison are far from clear; 

 it is probable that the conditions of experiment do not 

 sanction the comparison at all. But it might be that 

 the concentrations of the bodily contents are inversely 

 proportional to the losses by shrinkage. 



Any interpretation of the mechanism governing 

 shrinkage in solutions would involve assumptions. The 

 usual conclusion is that no solute can penetrate the 

 boundary between body and medium, and that the 

 water is then distributed between the two in propor- 

 tion to the relative osmotic pressures of body and of 

 medium. The data also allow the probability that 

 other forces are present, namely, forces such as main- 

 tain considerable concentrations of salts and organic 

 compounds inside the bodies when the medium has 

 zero concentration. 



The effects of strong solutions of sodium chloride, 

 of urea, and of sucrose upon body size are important 

 in themselves. But they are certainly not factors 

 which explain the differences between aged culture 

 medium and new culture medium. 



Acidity. Hydrogen ion concentration is known to 

 affect unicellular organisms in various physiological 

 respects. It is abundantly known that this concentra- 

 tion changes significantly with time in most culture 

 media. Its effect upon body size has been studied in 

 one species by Chalkley ('29). 



Body volumes in Ameba proteus were measured by 

 Chalkley, either by calculation from the areas of in- 

 dividuals viewed simultaneously from two angles, or 

 by measurement of the lengths of individuals drawn 

 into capillary tubes of known bore. In solutions of 

 lactose, glycerol, and urea which were certainly hyper- 

 tonic, body volumes always diminished, the changes 

 increasing with concentration and with time. In 

 Ringer's solutions buffered to various hydrogen ion 



