68 



Regulation of Size in Unicellular Organisms 



projected surface rather than to variability of body 

 volume, and for this reason the data need not here be 

 analysed further. 



<T3 

 ■ft 130 



a3 120 



CO 



110 



o 

 $- 



CD 



E 

 "6 100 



c 



SO 







4 



a 10 12 14 16 18 



Time in months 



Figure 30. Inheritance of diameter of shell, in a clone of Arcella polypora. 

 At A selection of the largest and of the smallest individuals was started, 

 seven selections being made up to B, when selection was stopped. At C 

 one unusually small individual was isolated, and the smallest of its progeny 

 was selected after each series of measurements up to D, when selection 

 was stopped. (Data of Jollos, '24). 



Difflugia. A very extensive study of size was made 

 in the species Difflugia corona by Jennings ('16). Vari- 

 ation of shell diameter was usually large in wild popu- 

 lations and in wild cultures, but very small (3.16 to 

 5.74) in pedigreed clones. The latter was not always 

 the case, for in the largest family studied the coeffi- 

 cient of variation was 13.65. In ordinary pedigreed 

 cultures, size differences were greater among distant 

 relatives than among close ones, and diversities of size 



