6 Regulation of Size in Unicellular Organisms 



Further, differentiations of other sorts, both struc- 

 tural and functional, are, it seems, inseparably con- 

 nected with the living state. It is apparent, therefore, 

 that the mechanisms of size regulation can never be 

 studied in homogeneous units. The "inclusions" are 

 integral parts of any protoplasm. 



A minute study. Those unicellular organisms that 

 are most complicated in visible structure are of course 

 the larger species. It is the larger types, such as cili- 

 ated protozoa, that have inevitably been most studied 

 with respect to size. Yet the individuals even of these 

 kinds are extremely small; it takes a million of the 

 largest race of Paramecium to fill a volume of one cubic 

 centimeter. All the unicellular individuals that have 

 ever been measured, and that are likely to be measured 

 for some years to come, would not occupy a volume 

 equal to these pages. 



2. Metabolic Turn-Over 

 When the dimensions of a unicellular organism are 

 ascertained, there are lumped together in one measure- 

 ment a large variety not only of structures, but as well 

 of chemical substances. Unicellular organisms, like 

 larger organisms, and probably to a greater extent 

 than larger organisms, are very variable in chemical 

 composition. 



Water. As of nearly all organisms, water is their 

 chief constituent. In Paramecium caudatum, water is 

 said, by Grobicka and Wasilewska ('25), to constitute 

 87.4 to 91.6 per cent of the body substance. Among 

 yeasts and bacteria, where many analyses have been 

 made, the values range from 68 to 91 per cent. (Bu- 

 chanan and Fulmer, '28). A. I. Dawson ('19) found 

 that the water content of a certain strain of Bacillus 

 coli, cultivated on an agar medium, changed tremen- 

 dously with the age of the culture, as is shown in figure 



