* Cultures in synthetic medium. 



It will be seen that the values of the mutation rate obtained by the second 

 method are all higher than the value found by the first method. This dis- 

 crepancy may be traced back to the same cause as the discrepancy between 

 the calculated and observed values of the standard deviation of the numbers 

 of resistant bacteria. This, we found, was due to an excess of early mutations, 

 giving rise to big clones of resistant bacteria. These big clones do not affect 

 the mutation rate calculated by the first method, but they do affect the results 

 of the second method, which is based on the average number of resistant bac- 

 teria. 



One sees in table 4 that the mutation rate calculated by the second method 

 does not vary greatly from experiment to experiment. In particular, it will be 

 noted that there is no significant difference between the values obtained from 

 cultures in broth and from cultures in synthetic medium, notwithstanding the 

 considerable difference of metabolic activity and of growth rate of the bacteria 

 in these two media. This shows that the simple assumption of a fixed small 

 chance of mutation per physiological time unit is vindicated by the results. 

 It may also be noted in table 4 that there is no significant difference between 

 the mutation rates obtained from 10 cc cultures and those obtained from .2 cc 

 cultures, or between the experiments with many and those with few cultures. 

 The variability of the value of the mutation rate seems to be solely due to the 

 peculiar probability distribution of the number of resistant bacteria in series 

 of similar cultures predicted by the mutation theory. 



At this point an experiment may be mentioned by which it was desired to 

 find out whether or not mutations occur in a culture after the bacteria have 

 ceased growing. A culture was grown to saturation and was then tested re- 

 peatedly for resistant bacteria and for total number of bacteria over several 



