A study of bacterial populations with induced nuclear and cellular divisions 



at the same rate as the total cell counts and the lysogenization frequency assumes a 

 constant value which is slightly higher than that characteristic of growth at 25 C. 

 The insert in Figure 3 shows the changes in lysogenization frequency during the first 

 30 minutes; it should be added that the time after the change of temperature when 

 the abrupt rise occurs is very constant from one experiment to another and that the 

 rise is always close to two-fold. 



20 



40 



Figure 3. Growth and lysogenesis curves as influenced by 

 a single temperature shift. Open circles {and upper curve) 

 show counts of ly so genie colonies ; points [and lower curve) 

 represent total colony counts. The slopes of the early por- 

 tions of the curves are determined by counts made during 

 the 90 minutes preceding the temperature shift. As in 

 Figures la and \b the distance between horizontal lines 

 corresponds tc a doubling of cell counts. The insert in this 

 figure illustrates the changes in the lysogenization frequency 

 during the first 30 minutes of the 37 C. period, and should 

 be compared with the continuous curve of Figure 2. 



Before attempting to draw a comprehensive picture of the various effects of 

 temperature changes on cell division and lysogenization we shall present the results 

 of cytological studies made on samples taken during an experiment similar to the 

 one presented in Figure 3. First a few words must be said about the fixing and staining 

 techniques employed. Portions of culture were withdrawn and at once mixed with a 

 solution of Os0 4 in buffered saline to give a final concentration of Os0 4 of o 2 per 

 cent. ; after centrifugation the pellets were resuspended in small volumes of a o 2 per 

 cent, solution of Os0 4 , and 20 minutes after the first mixing with Os0 4 loopfuls of 

 the concentrated suspensions were spotted on dry agar surfaces. This somewhat 



165 



