The strains By and Bal have been called A and C in previous papers (DELBRUCK and LURIA 1g4z), in which the interest was focused on their use as indicator strains for plating mixed virus suspensions rather than on their mutational origin. 3 Several of the experiments with virus y' were done using a strain Byaz instead of strain By, because our stock of strain By (obtained from strain B in 1941) had recently, in the course of subculturing, become partially sensitive to virus y. Strain Bycuz,

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... ed from By before this occurred, behaved toward viruses y and 7' exactly like the original strain By.