EMORY L. ELLIS AND MAX DELBRUCK 377 



shows. 0.1 cc. of phage of appropriate and known concentration was 

 added to 0.9 cc. of an 18 hour culture and allowed to stand in this con- 

 centrated bacterial suspension for 10 minutes at the temperature of 

 the experiment. This mixture was then diluted 1 : 10* in broth and 

 incubated at the temperature chosen. Samples of this diluted mix- 

 ture were withdrawn at regular intervals and assayed. The results 

 of three such experiments are plotted in Fig. 3. The rise corresponds 

 to the average number of phage produced per burst, and its value can 

 be appraised better in these experiments than in the complete growth 

 curve previously given (Fig. 2) where there is probably some over- 

 lapping of the steps. In these experiments the rise is seen to be 

 practically identical at the three temperatures, and equals about sixty 

 particles per infected bacterium, but the time at which the rise oc- 

 curred was 30 minutes at 37°, 60 minutes at 25°, and 180 minutes at 

 16.6°. This shows that the effect of temperature is solely on the 

 latent period. 



We have also made separate measurements of the rate of bacterial 

 growth under the conditions of these experiments. They show that 

 the average division period of the bacteria in their logarithmic growth 

 phase varies in the same way with temperature, as the length of the 

 latent period of phage growth. The figures are: 



There is a constant ratio (3/2) between the latent period of phage 

 growth and the division period of the bacteria. This coincidence 

 suggests a connection between the time required for division of a 

 bacterium under optimum growth conditions, and the time from its 

 infection by phage to its lysis. 



Individual Phage Particle 



The growth curves described above give averages only of large 

 numbers of bursts. They can, however, also be studied individually, 

 as was first done by Burnett (11). 



49 



