136 Regulation of Size in Unicellular Organisms 



stances. It was only after the more or less successful 

 device of Jennings ('08b) that pure lines of holotrophic 

 organisms under experimental and control conditions 

 could legitimately even be compared while being so 

 cultivated. His device was to cross-inoculate daily all 

 parallel cultures with a mixture of the bacteria present 

 in each culture. Thus the culture media were much 

 more similar to one another than by earlier procedures ; 

 nevertheless the variations from day to day were tre- 

 mendous. In spite of improvements to date, such as 

 those of Hartmann ('21) it remains true of all species 

 of unicellular organisms that, as postulated by Jen- 

 nings ('08b) "a small mass of fluid constitutes a rela- 

 tively large and varied environment for Paramecium. 

 A watch-glass of hay infusion is a microcosm to this 

 animal." 



The studies now to be reviewed are mere beginnings 

 at the analysis of chemical conditions. In future days 

 when media can be fully controlled they will look crude 

 indeed; yet they serve at least to demonstrate the ex- 

 istence of inconstancy of cultural conditions. 



Ciliates. Pearl ('06) studied the variability of size 

 in wild cultures of Chilomonas Paramecium. This 

 species has a fairly constant elongated ovoid form. It 

 reproduces by binary fission; but division is longitud- 

 inal, not transverse as in Paramecium, so that growth 

 in length is not a marked feature. Normal frequency 

 curves of variability were found upon fixed prepara- 

 tions made at a single time, and the co-efficients of 

 variation for length and for breadth were 10.3 and 10.9 

 respectively; about the same therefore as in Parame- 

 cium from wild cultures. Pearl compared Chilomonas 

 taken at two times from the wild culture, once when 

 food was abundant and multiplication was rapid, and 

 once when circumstances were "unfavorable." In the 

 latter case the body was smaller both in length and in 



