VICTOR G. BRUCE 131 



suits of one such experiment. A culture of N. tt/p)iiiniiriiim growing ex- 

 ponentially in synthetic medium at 37° has a generation time of about 

 45 minutes as (ietennincd fi'oni (•ur\-(' A in the figure. At zero time 10~"* 

 , M 5MT was added to a portion of this exponentially growing culture and 

 at t = 25 minutes excess try])tophane was added to a portion of the 

 culture containing 5MT. In this and in other similar experiments it was 

 noted that 5jMT initially inhibits cell division as well as growth but that 

 subsequently the cells divide once in the presence of 5MT. The addition 

 of excess tryptophane at a time when a large fraction of the cells are 

 newly divided results in an increase in turbidity without a corresponding 

 increase in colony counts. The jump in the colony count curve, occurring in 

 the presence of 5MT, also occurs if the concentration of the 5MT is ten 

 times higher. At the higher concentration of 5MT the turbidity increases 

 even less than in the above-described experiment and the average cell size 

 is correspondingly less. 



There is thus evidence that in at least one respect the description of the 

 cell division cycle is similar in bacteria and protozoa. The implication that 

 there is a point during the later part of the cell cycle at which the cell 

 enters more or less irreversibly into division, and that the processes leading 

 up to this point are sensitive to being blocked by different methods, pro- 

 vides a natural explanation for some of the synchronization systems to 

 be described. 



SYSTEMS IN WHICH SYNCHRONIZATION HAS BEEN ACCOMPLISHED 

 BY MEANS OF TEMPERATURE EFFECTS 



Although some synchronization effects had been observed before 1953, 

 it was not until 1953 and 1954 that workers began to develop synchroniza- 

 tion methods as a means of invesigating the division cycle. Hotchkiss 

 (6), Zeuthen and Scherbaum (18, 23), and James (9) independently 

 decided on temperature changes as a means of inducing synchrony, and 

 each used a somewhat different method. Maal0e and Lark (10, 13) were 

 led to try temperature changes as a result of the work of Hotchkiss, and 

 of Zeuthen and Scherbaum, and they also independently developed still 

 a different method. Subsequently, Szybalski and Huntcr-Szybalska (19) 

 used a method very similar to that used by Hotchkiss. 



Lark and Maal0e (10, 13) have investigated the effect of single and of 

 multiple temperature shifts on the growtli of cultures of S. typhimurium. 

 They find that a culture growing exponentially at 37° will continue growing 

 exponentially at a slower rate if the temperature of the culture is reduced 

 to 25°. However, if a culture is growing exponentially at 25° and the tem- 

 perature is raised to 37° the culture continues growing at the 25° rate for 

 about 20 minutes, following which the growth rate is for a short time 



