EMORY L. ELLIS AND MAX DELBRUCK 



371 



efficiency of plating and not from a quick increase in the number of 

 infective centers is evident from the following experiment. Bacteria 

 were grown for 24 hours at 25°C. on agar slants, then suspended in 

 broth. Phage was added to this suspension and to a suspension of 

 bacteria grown in the usual way, and the concentration of infective 



a- 



Min.O 



20 



40 



60 



80 



100 



Time 



120 



MO 



150 



180 



200 



Fig. 2. Growth of phage in the presence of growing bacteria at 37°C. 

 A diluted phage preparation was mixed with a suspension of bacteria con- 

 taining 2 X 10* organisms per cc, and diluted after 3 minutes 1 to 50 in broth. 

 At this time about 70 per cent of the phage had become attached to bacteria. 

 The total number of infective centers was determined at intervals on samples of 

 this growth mixture. Three such experiments, done on different days, are plotted 

 in this figure. The same curve was easily reproducible with all phage prepara- 

 tions stored under proper conditions. 



centers was determined on both. The initial value was 1.6 times 

 higher in the agar grown bacteria than in the control experiment, and 

 remained constant until actual growth occurred. The initial rise was 

 therefore absent in this case, clearly a result of an increase in the 

 efficiency of plating. A sufficient number of experiments were per- 



43 



