538 ERIK ZEUTHEN 



the subsequent yl hr. the temperature is shifted eight times, and in a 

 regular manner, between 29"" C. and 34"^ C. This blocks cell multiplication. 

 The lower curve (division index = cytokinetic cells /total cells) shows that 

 in response to the temperature changes cells in division complete this 

 process while no new cells enter into division. At subsequent constant 

 29° C. almost all cells divide at the same time, but only after a delay of 

 1 3 hr. The cells complete this division together and they enter a second 

 and a third division at 2-hr. intervals. This is reflected both in the popula- 

 tion counts and in the division indices. In Fig. 2 the fully-drawn curve 

 repeats and extends the broken curve from Fig. i. Only the counts are 



5-0 



4-5- 



40 



3-5 



^4.5 



-40 



-3 5 ^ 



-3 



2-5 



10 



14 

 (hours) 



22 



26 



Fig. 2. The continuous curve represents cell counts, the broken curve repre- 

 sents volume of cells per unit volume of culture. (From Zeuthen and Scherbaum 



[14])- 



not spaced close enough to describe the synchronous division steps. In 

 Fig. 2 the broken curve is for total cell volume per aliquot. During the 

 phase of shifting temperature the average cell grows to about three times 

 its average logarithmic size. This is reversed during the synchronous 

 division steps which occupy about 5 hr. as indicated. The relative position 

 and the slopes of the two curves suggest complete reversibility of the 

 induced division synchrony. Cells which through synchronous division 

 have reverted to logarithmic growth may be resynchronized by the method 

 shown in Figs, i and 2. The cells which after the heat shocks make ready 

 for their first synchronous division are abnormally large, not only by 

 volume but also by dry matter, protein content, RNA, DNA and by 

 nuclear volume [2, 1,7, 14, 12 (review)]. All measures increase more or 



