SYNCHRONIZED GROWTH IN TETRAHYMENA CELLS 143 



cessive synchronous divisions suggests that the system is fully balanced 

 only when, after several generations, it has reverted to logarithmic 

 growth. In a large material the time (at room temperature) from E.H. 

 to the first division maximum was 99 ± 12.6 minutes. The second divi- 

 sion maximum came 127 ± 14.5 minutes after the first, the third 110 ± 

 20 minutes after the second, and the fourth 135 ± 8.8 minutes after 

 the third. ( Medium C, temperature cycles as in Figure 4.) 



Physiological mechanisms in the induction of synchrony 



Synchrony is not obtained because the rate of advance towards 

 division is differently influenced in cells of different ages. On the con- 

 trary (Zeuthen and Scherbaum, 1954; Thormar, 1959) all temperature 

 changes, even the smallest, make a cell lose, not gain, time in its 

 preparation for its next division. 



Take two logarithmic cells which have completed their previous 

 division at the same time. Let the first go on through the next division 

 at the same temperature (28-29^ C.) at which both were reared. At 

 100 to 110 minutes after a division, expose the second cell to a new 

 temperature for a short time (say 30 minutes) and return to 28° C. 

 The second cell will always be, or tend to be, delayed. If the new 

 temperature is around 22° C. or about 30° C, the treated cell makes 

 no advance in time at all at this temperature. It therefore divides 

 exactly 30 minutes later than the control. If the cell is exposed for 30 

 minutes to temperatures in the interval 22-30° C, it advances some, 

 but less than one would have expected on examination of the relation 

 between temperature and growth over generations at the different 

 temperatures. The cell is thus delayed, but less than 30 minutes. If ex- 

 posure is made to temperatures below 22° C. or above 30° C, the delay 

 is more than the 30 minutes spent at the new temperature, sometimes 

 much more. Thus (cf. Figure 7), depending on the shock temperature, 

 the cell appears to be either slowed (22-30° C), blocked (22 or 30° 

 C), or more severely influenced ( <22° C. or >30° C). In the latter 

 case, time for reconstitution at constant 28° C. is required. We have 

 used the neutral terms "excess-delay" (Thormar, 1959) or "setback" 

 (Zeuthen, 1958) for this time. We might simply call it the "repair 

 time," or "recovery time," for the division-preparing machinery. 



When Thormar studies the course of events at any new tempera- 

 ture to which the 100-110-minutes-old logarithmic cells are transferred, 

 he gets results which are illustrated in Figure 8. With increasing length 

 of exposure to any of the temperatures represented, the setback, or the 

 recovery time at 28° C, first (31° C. and 33° C.) increases to a maxi- 

 mum and then slowly again decreases. At 34.1° C. the setback in- 

 creases, initially as fast as at the lower temperatures, but the increase 



