542 ERIK ZEUTHEN 



in Tetrahymena cells, synchronized as well as normal cells from logarithmic 

 cultures. ^-FPhe is antagonized competitively by phenylalanine. In 

 principle we have found no difference in response between synchronized 

 cells and cells from a logarithmic population. The response of the former 

 cells is only much more easily analyzed than that of the latter, so our work 

 on logarithmic Tetrahymena cells has mostly served as a control on results 

 obtained with the synchronized cells. 



Figure 4 shows two combined experiments for cells which have been 

 synchronized and remain in proteose-peptone (0-4% liver fraction). The 

 two upper curves show the three first synchronous divisions (1,2, and 3) 



0.5^ 



00 - o o oral 



MOI O O O O O Q 0000 qK5) 



II 



in 



60 



-J^o-< 







60 



- (Ol — o o o - 



180 



120 180 240 300 



Minutes after E H. 



360 



Fig. 4. Upper curve: Division maxima 1-3 in control represented by the 

 changes in time of the division index. Lozver curves: Delays of divisions 1-3 (curves 

 I-III) as a function of the time of im.mersion of the cells into/)-FPhe, 16 mM in 

 proteose-peptone. 



in the main controls. Divisions appear as maxima on the curves for the 

 division index. The inhibitor is 16 niM /)-PThe. It is added to the cells for 

 continuous exposure at the times (abscissa) indicated by the position of 

 the points on curves I, II and III. The ordinate of a point represents the 

 delay (relative to the parallel control) of division i (curve I), division 2 

 (curve II), and division 3 (curve III). The infinity sign indicates block of 

 the subsequent division. All observations are at 28° C. 



Obviously, there is a critical time before a division when a decision is 

 made whether or not that division can be blocked by the amino acid 

 analogue. This time (interpreted as shown in Fig. 4) is 42 mins. before 

 division i, 40 min. before division 2, and 47 min. before division 3. We 



