parable to its titer of virus a. In our experiments all these possibilities were 

 actually realized: we found several cultures with growth of B«i. one culture 

 with growth of Ba 2 , and one culture completely lysed but containing such a 

 large amount of virus a' as to suggest that virus a' had multiplied on a sec- 

 ondary growth of Ba2. 



After incubation, the completely lysed cultures were tested for the amount 

 of virus a (or virus 7) and for the number of plaques produced on a-resistant 

 (or 7-resistant) bacteria. For the latter purpose, the whole contents of each 

 culture were plated according to the technique devised by Gratia (1936b) and 

 by Hershey et al. (1943). The results are given in tables 3 and 4. The amounts 

 of virus a (or virus 7) represent averages; the individual counts of normal 

 viruses in different cultures within each experiment never vary more than can 

 be accounted for by the sampling errors. In considering the counts from the 

 platings with the resistant bacteria, we must remember that for both virus a' 

 and virus 7' the efficiency of plating is low and slightly variable from one ex- 

 periment to another. The plaque counts should be too low by a factor corre- 

 sponding to the efficiency of plating. It is worth recalling that this factor, if 

 constant within one experiment, should not affect the type of distribution if 



230 



