Induced Mutational Changes in Yeast 109 



of fermenting sucrose. If, in the mass of cells, there had been 

 even one cell capable of fermenting sucrose, fermentation 

 would have begun and would have been evident not later 

 than 24 days after contact with sucrose (see Table I). 



The two cultures, which began fermenting sucrose after 

 having been maintained on lactose, should be considered to 

 have become adapted to fermentation of sucrose during the 

 period of their cultivation on sucrose, i.e. due to the influence 

 of sucrose. The results of this experiment are summarized 

 in Table II. This table shows that the ability of yeast cells to 

 ferment sucrose is associated with the presence of this sugar 

 in the medium. 



Experiment 2. The previous experiment showed that the 

 addition of glucose to the medium leads to some acceleration 

 of adaptation of experimental cultures to fermentation of 

 sucrose. It was of some interest to do the experiment without 

 adding glucose to the medium. In contradistinction to 

 Expt. 1, one diploid culture of S. globosus (obtained from a 

 single spore) was used. The culture was seeded into Petri 

 plates containing wort-agar. The 2 -day colonies were re- 

 seeded into test-tubes, with gas-traps, containing a medium 

 which consisted of 4 per cent sucrose and • 5 per cent auto- 

 lysed yeast extract. Altogether, 454 test-tubes were seeded 

 with the same number of colonies. The aim of this experiment 

 was to demonstrate spontaneous mutation in response to 

 sucrose-fermenting capacity during the reproduction of cells 

 on wort-agar on Petri plates. The experiment was carried out 

 over a period of 160 days. During this period, out of a total 

 of 454 cultures only 5 started to ferment sucrose ; the first one 

 on the seventy-seventh day, and the other four on the eighty- 

 sixth, 120th, 122nd and 122nd day, respectively. Taking into 

 account the data shown in Table I, this experiment can be 

 said to confirm completely the previous ones, and it demon- 

 trates that, out of a great number of cultures, here again not 

 a single case of sucrose fermentation can be explained by the 

 theory of spontaneous mutation. 



It should be pointed out that cells which do not ferment 



