SYNCHRONIZED GROWTH IN TETRAHYMENA CELLS 



139 



Figure 4. Relation between 

 the number of temperature 

 shocks appUed and the per- 

 centage of cells participating 

 synchronously in the first, sec- 

 ond, and third divisions. Me- 

 dium for A: 2 per cent pro- 

 teose-peptone. For B: Same 

 plus 0.1 per cent liver fraction 

 L. For C: Same plus 0.4 per 

 cent liver fraction L. Tem- 

 perature shocks, each of 20 

 minutes to 34° C, were sep- 

 arated by somewhat extended 

 periods (40 minutes) at 28° 

 C. Division was at constant 

 28° C. 



5 10 15 



temperature shocks 



Figure 5. The same experi- ^ 



ments as in Figure 4. The ^J00<^ ,,.. .- J— ^ 

 time intervals indicated are "^ 

 from "end of heat treatment" 

 (E.H.) to division 1, from di- 

 vision 1 to 2, and from divi- 

 sion 2 to 3. E.H. is the time 

 when the clock switches the 

 control from the 34° C. to the 

 28° C. thermometer. 



5 W 15 



temperature shocks 



to longer-sustained synchrony ( three, four, five divisions, of. later Fig- 

 ure 12 ) than the poorer media do. 



Sooner or later temperature cycling, if continued, leads to poorer 

 synchrony. This has tv^^o causes ( of. Figure 6 ) : some of the cells adapt 

 to, and divide (3, 4) at the cycling temperature, others (7) do not 

 divide until after many (perhaps 24) hours at constant 28° C. This 

 latter effect is a function of excessive growth without division and of 



