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S. E. LURIA AND M. DELBRtJCK 



rate per bacterial generation is independent of the physiological state of the 

 bacteria may be too simple. If the mutation rate is higher for actively growing 

 bacteria than for bacteria near the saturation limit of the cultures, early muta- 

 tions and big clone sizes will be favored, and therefore higher variations of the 

 numbers of resistant bacteria can be expected. Second, the assumption of a 

 sudden transition from sensitivity to resistance may also be too simple. It is 

 conceivable that the character "resistance to virus" may not fully develop in 

 the bacterial cell in which the mutation occurs, but only in its offspring, after 

 one or more generations. However, if this were the case, cultures with only one 

 or two resistant bacteria should be relatively rare. The last experiment listed 

 in table 3, in which the entire cultures were plated, shows a rather high propor- 

 tion of cultures with only one resistant bacterium. This seems to show that the 



Figure 2. — Experimental (Experiment No. 23) and calculated distributions of the numbers 

 of resistant bacteria in a series of similar cultures. Solid columns: experimental. Cross-hatched 

 columns: calculated. 



character "resistance to virus" in general does come to expression in the bac- 

 terial cell in which the corresponding mutation occurred, as assumed by the 

 theory. 



Another way of comparing the experimental results with the theory is to 

 compare the experimental distribution of resistant bacteria with the approxi- 

 mate distribution calculated by the method outlined at the end of the theo- 

 retical part. The theoretical distribution has to be calculated from the aver- 

 age number of mutations per culture given by equation (5). Only experi- 

 ments where the whole culture is tested can therefore be used for such a 

 comparison. This method tests the fitting of the expectations for small numbers 

 of resistant bacteria, in contrast to the comparison of the standard deviations, 

 which involves predominantly the cultures with high numbers of resistant 

 bacteria. 



Figure 2 shows the experimental and calculated distributions for Experi- 

 ment No. 23; the cultures with more than nine resistant bacteria are lumped 

 together in one class, since the distribution has not been calculated for values 

 higher than nine. 



It is seen that the fitting for small values is satisfactory. In particular, the 



