mutants were produced spontaneously but randomly in time or whether the presence of the phage induced a physiological change that caused resistance. Luria reasoned that if mutations occurred sponta-neously, then the mutations might be expected to occur at different times in different cultures; so the resulting numbers of resistant colonies per culture should show high variation (or “Fuctuation” in his word). He later claimed that he obtained the idea while watching the Fuctuating returns obtained by colleagues gambling on a slot machine at a faculty dance in a local country club; hence the term “ jackpot ” mutation . Luria and Delbrück designed their “fuctuation test” as follows: They inoculated 20 small cultures, each with a few cells, and incubated them until there were 10 8 cells per milliliter. At the same time, a much larger cul-ture also was inoculated and incubated until there were 10 8 cells per milliliter. The 20 individual cultures and 20 samples of the same size from the large culture were
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