36 L. L. Cavalli-Sforza 



Table I shows the results obtained with a saturated culture 

 of Esch. coli Kl2 no. 176 which was expected, from the 

 assay, to contain zero streptomycin-resistant mutants. Three 

 samples from it (of 0-5 ml., 0-25 ml. and 0-125 ml., respec- 

 tively) were diluted each to 20 ml. with fresh broth. Each 

 20-ml. quantity was distributed into 10 tubes, in quantities 

 of 2 ml. per tube, and the 30 cultures thus obtained were 

 incubated to saturation. Eventually the total number of 

 resistants per culture was counted on streptomycin agar 



Table I 



A Protocol of Quantitative Indirect Selection 



From a culture which has been assayed for streptomycin resistants and 

 found to contain zero resistants per ml. , further samples have given : 



Average per subculture 4-9 6-4 



6-3 



(500 (jLg./ml.). Some resistant cells were found (Table I), but 

 only a few, and often there were none per culture ; the average 

 number of mutants expected in such conditions from more 

 extensive tests is 4-8 ±2-0 per culture, and the results from 

 the three series agree within the limits of error with this 

 number. 



Table II shows what happened instead when a culture 

 known to contain 39-5 resistant cells (from the assay of a 

 sample from it plated in streptomycin agar) was treated in 

 the same way. Here the sample of 0-125 ml. was expected 

 to contain 5 mutants and if, on subdivision into 10 tubes, 

 each of these fell into a separate tube, 5 out of these would 



