476 HOWARD B. NEWCOMBE 



first division. This phenomenon, although of interest in itself, does not affect 



the main conclusions. 



It is estimated that phenotypic expression occurs in at least one member of a 

 mutant clone approximately two or three generations after the mutation, and 

 that in many cases it affects at first only one individual, others of the mutants 

 becoming resistant during subsequent divisions. 



Expression does not appear to be delayed in any appreciable proportion of a 

 clone beyond approximately six generations after the mutation. 



ACKNOWLEDGEMENTS 



I wish to thank Dr. M. Demerec for much helpful advice and criticism, 

 Dr. B. McClintock for valuable discussion of the work during its progress 

 and Dr. S. E. Luria for a number of very useful comments on the manuscript. 



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78 



