56 Regulation of Size in Unicellular Organisms 



constancy of mean body length in one clone for more 

 than six years. 



Selection. The next step was to see if large or small 

 individuals could be selected, and possibly give rise 

 to lines with new size characters. Jennings ('08b) 

 showed that whereas selection in a wild culture might 

 isolate distinct size biotypes, selection within a clone 

 had no effect upon the sizes of the descendants. The 

 variability found within a pure line could not be fur- 

 ther reduced or segregated. Within those few gen- 

 erations after selection had ceased which were neces- 

 sary for securing enough individuals upon which to 

 make measurements of significant frequencies, the 

 Paramecia had reverted exactly to the original size 

 distribution. This fact was confirmed by Jollos ('13, 

 '21) andAckert ('16). 



Interposed influences. Further purposely inter- 

 posed environmental effects did not interfere with the 

 transmission of size constancy throughout the genera- 

 tions of Paramecium. Jennings ('08b) showed that 

 starvation, old culture fluid, rich culture fluid, deep cul- 

 ture fluid, all caused size variations. But restoration 

 to basal circumstances resulted in restoration of size 

 within the time required for the breeding of enough in- 

 dividuals to measure. Rautmann ('09) and Jollos ('13, 

 '21) found that temperature changes modified the di- 

 mensions of Paramecium. But the latter investigator 

 showed that in some clones either holding at the new 

 temperature for some weeks or restoring to the origi- 

 nal temperature resulted in full return to the original 

 characteristic size. DeGaris ('27) found that gross 

 bodily abnormalities and temporary inhibition of re- 

 production, produced by treatment with cyanide, did 

 not affect the transmission of the size character in 

 Paramecium. 



Inheritance and its expression. It is evident that, 



