146 Regulation of Size in Unicellular Organisms 



cultural ages can his results not be adequately treated 

 here. Suffice it to point out that the lowest variabili- 

 ties occurred slightly after the maximal sizes had been 

 attained; this might be taken to mean that the individ- 

 ual and the race were under most nearly "normal" con- 

 ditions when it was of largest size, was assimilating 

 fastest, and was reproducing fastest. 



The relation between body size and the maximum 

 population attained in a uniform culture medium was 

 studied by Butterfield ('29), with the important out- 

 come that the total bulk of protoplasm was approxi- 

 mately uniform for diverse species. This seems to 

 mean that any one of five species of bacteria or the 

 ciliate Colpidium, which were the kinds measured, 

 would multiply until the food material had diminished 

 to some definite small amount. The material utilized 

 would yield the same mass of protoplasm whether 

 transformed by one species or another. 



Experiments concerning the influence of the volume 

 of the medium and of the ratio of the number of organ- 

 isms to the volume and surface of the medium have not 

 been performed in such a way as to distinguish fully 

 between nutritional, physical, and specific chemical 

 factors. The influence of these factors upon size has 

 hardly been analyzed even to the extent of finding 

 whether such factors exist. It might be assumed that 

 one organism in a cubic millimeter of fluid is equivalent 

 to a thousand individuals in a cubic centimeter of fluid, 

 with respect to size and fission rate. But is it true that 

 such is the case? So far it can be stated only that the 

 contrary has never been satisfactorily observed. 



4. Chemical Environment 



Are those modifications in organisms which result 

 from ageing of the culture medium entirely due to 

 changes in the food supply, or can significant influ- 



