Induced Mutational Changes in Yeast 105 



cells. The fourth variant was the control, where no sucrose- 

 nonfermenting cells were present. After the sucrose-non- 

 fermenting cells had been introduced into the test-tubes, one 

 sucrose-fermenting cell was added to each test-tube of each 

 variant (20 test-tubes per variant). Observations of fermenta- 

 tion of experimental and control cultures were continued for a 

 period of 45 days. A second series of experiments was carried 

 out along the same lines. The results of both series of experi- 

 ments are shown in Table I. 



Table I shows that the increase in concentration of sucrose- 

 nonfermenting cells corresponds with an increase in the time 

 which elapsed between the addition of one sucrose-fermenting 

 cell and the appearance of the first signs of fermentation. If 

 in the absence of sucrose-nonfermenting cells fermentation 

 begins on the third or fourth day, then in the presence of 

 sucrose-nonfermenting cells we observe a retardation of 

 fermentation by approximately 15-20 days. In one case, 

 fermentation commenced only on the twenty-fourth day. 

 However, no very long delay was noted although observations 

 were continued over a period of up to 45 days. 



Experiment 1, The above data demonstrate that in order to 

 see spontaneous mutation, a 10-day period of observation of 

 the process of fermentation is not long enough. In the condi- 

 tions of our experiments the maximal period was 24 days. 

 The above results were taken into account in our first experi- 

 ment, and we increased the content of sucrose in the medium 

 to 6 per cent, adding • 75 per cent glucose to the sucrose. It 

 was suggested that, firstly, the yeast cells were propagating 

 and fermenting glucose to form a fermentative type, and that 

 thus the transition to sucrose-fermenting cells was made 

 easier — provided these cells can produce invertase. Secondly, 

 the increased sucrose concentration in the medium could also, 

 to some extent, lead to an increased effect on variability — 

 provided that the presence of this sugar in the medium stimu- 

 lates the formation of invertase by the cell. A medium con- 

 taining 6 per cent lactose and • 75 per cent glucose was also 

 prepared. The number of cultures used in the experiment was 



