116 Regulation of Size in Unicellular Organisms 



also that conjugants are not obtained under the same 

 environmental conditions as the most healthy non- 

 conjugants. 



Observations more nearly of the sort indicated were 

 made by Jollos ('13, '21) on Paramecium. In one in- 

 stance he showed that after four conjugations had in- 

 tervened the vegetatively reproducing clonal popula- 

 tion had the same size distribution as that originally 

 present. In these observations, it may be said, time 

 was allowed for the genotypic constitution to express 

 itself as phenotype. In Jennings's observations, of 

 course, it seems as though 25 days would allow enough 

 time for any variants to come back to normal in their 

 descendants. 



If Jennings's data are to be taken to indicate genetic 

 segregation among the ex-conjugants belonging to a 

 clone, it would be necessary to rule out the effects of 

 conjugation upon the manifestation of size in all the 

 lines studied. 



Size mutation. In two cases, Jollos ('13, '21) ob- 

 served sudden changes of size within mass cultures 

 which were clones (figure 33). Jollos considered it 

 impossible that these could be due to contaminations, 

 and so he believed them to be mutations of the geno- 

 type. One size-mutant race differed in susceptibility 

 to temperature and to arsenious acid as well as in size. 

 The altered sizes were likewise accurately transmitted 

 through at least three subsequent conjugations. This 

 is the type of occurrence which every biologist would 

 like to witness. 



In both of the mutations which Jollos observed the 

 size jumped to a smaller value. It is perhaps of in- 

 terest that the modal body length of the mutant line b 

 (figure 33) is 80 per cent of the length of the original 

 line a. If all the linear dimensions are proportional, 

 a reduction of 20.6 per cent in them is equivalent to a 



