272 



H. H. PLOUGH 



dosages, and some of the values are less significant statistically, but the per- 

 centage of mutants is significantly higher at the higher dosages. This is em- 

 phasized by lines IX and X where the sums of the first three and the last 

 four values are compared. The same conclusion is evident from inspection 

 of column 7 in the table, where the numbers of different mutants at the 

 successive dosages are shown. Nearly three times as many were isolated from 

 the upper group as from the lower. 



TABLE 17.2 

 KINDS OF AUXOTROPHIC MUTATIONS IN S. TYPHIMURIUM 



Line XI in the table shows the result of one radiated series made on a 

 different initial strain, #519. Comparison of the column 5 and column 7 

 totals with line VII above, shows that this strain is much more resistant to 

 radiation than is strain #533. It is clear that comparisons of the mutagenic 

 effects of radiation dosage must always be made between samples from the 

 same strain. 



The data in Table 17.1, column 5, are graphed in Figure 17.3. Compari- 

 son of the percentages of mutants at successive dosages shows a positive 

 correlation, though rather far from a straight line curve. As the penicillin 

 screening method involves a 24 hour growth in complete broth, and another 

 24 hours in minimal medium with penicillin, it might be expected that the 

 final percentage of mutant strains would not bear the direct relation to dos- 

 age shown in tests of mutations produced in germ cells in sexually reproduc- 

 ing organisms. Indeed Davis, in his account of the penicillin screening method 



