ERIK ZEUTHEN AND OTTO SCHERBAUM 



the whole culture into one group, we applied a series of heat shocks (33 5 C.) each of 

 half an hour with half an hour at optimum temperature (29-5° C.) in between. This 

 had the expected effect, as il'ustrated in Figure 5. During the intermittent heat treat- 

 ment cell divisions were continuously suppressed, but after return to 24° C. the vast 

 majority (85 per cent.) of all cells entered into division simultaneously. As two more 

 peaks of synchronous division were observed at 24° C. this group must have passed 

 through a total of three cell cycles at constant room temperature. This experiment 

 was repeated many times, always with the same result. It was of no significance whether 

 the cells were returned to constant optimum or to constant room temperature, and 

 we got more or less the same result whether we let the temperature cycle between 



Figure 6. The continuous curve represents division index, and the 

 broken curve represents cell counts. 



28° C. and 34 C. or between 29 5 C. and 32 C. Besides the well-synchronized 

 cycling there is one more striking thing to observe from Figure 5, namely, that the 

 intervals of time between the peaks represent only about 60 per cent, of the duration 

 of the cycle in normal cells. Whereas cell division was blocked during heat treatment, 

 after end of treatment it went on at a faster rate than usual. 



These preliminary results were checked with actual cell counts as demonstrated 

 in Figure 6. Multiplication was blocked during the period of cycling temperature, 

 and after the return to optimum temperature the cell count increased in the expected 

 step-wise manner, the height of every step approximately corresponding to a doubl- 

 ing in number. The division index was followed in the same experiment. Three 

 successive bursts of divisions were observed. 



148 



