46 A. D. HERSHEY AND RAQUEL ROTMAN 



virus for a differential count of its component types. This procedure was first 

 used, for another purpose, by Delbruck and Luria (1942). 



The advantage of the hXr cross is that all the genetic types of virus to 

 which it gives rise can be recognized in a single plating on a mixture of bac- 

 terial strains (fig. 3). This makes possible the analysis of viral yields from single 

 bacterial cells. For this purpose the procedure already described is modified 





Figure 3. — Progeny of the cross hX.fl plated on mixed indicator. The acentric clearings in 

 the h + r plaques result from secondary h mutations. 



by increasing the factor of dilution to obtain only one infected bacterium to 

 about three ml of nutrient broth. The culture is then divided up into samples 

 of one ml, most of which will contain either no bacteria at all, or a single one. 

 A large number of these samples are plated out after the bacteria lyse, and the 

 elementary yields, averaging about 500 particles of virus in our experiments, 

 are analyzed in toto. Both the mixed indicator method for differential counting 

 of viral mixtures, and the single burst technique as here employed, were first 

 used by Delbruck (1945a, b). 



The remaining portion of this discussion of materials and methods is of 

 technical interest only. 



Viral stocks are prepared by seeding nutrient broth cultures of E. coli strain 

 S with material taken directly from a single plaque. Stocks of wild type and 

 h mutant usually have titers exceeding 10 10 /ml; r mutant titers are always a 



240 



