66 



Regulation of Size in Unicellular Organisms 



Hegner ('19a) selected individuals of Arcella den- 

 tata which exhibited large and small sizes, and found 

 that then the diversities were maintained through 

 many weeks; and at least six weeks after selection had 

 ceased. He supposed that this diversity within a clone 

 was permanent. 



Generations 



Figure 28. Progression of shell sizes in two lines of Arcella dentata. Two 

 single uninucleate individuals were cut into two between the zero and first 

 generations, and all the progeny remained uninucleate throughout the ob- 

 servations. (Data of Hegner, '20b). 



Selection for shell size was carried out by Jollos 

 ('24) in three species of Arcella. He gives data only 

 for A. polypora; in this species cultures were followed 

 for eleven months after selections had ceased. Peri- 

 odically he measured the sizes of the progeny, but he, 

 like Hegner, does not state whether or not he counted 

 with the later progeny those immediate progeny of 

 parents whose shells had persisted from previous peri- 

 ods. Jollos's best experiments are represented in fig- 

 ure 30, and it is apparent that during many months 



