136 



CELLS, TISSUES, AND ORGANISMS 



After transfer to constant 28° C. or to room temperature, the block is 

 released after a well-defined lag. The population then goes through 

 one, two, or more consecutive divisions in first excellent, later deterio- 

 rating, synchrony ( Figure 1 ) . Provided the population density does not 

 get too high, the system gradually reverts to logarithmic growth with 

 smaller cells (Figure 2). It may be re-synchronized with the standard 

 procedure. The effects of the cycling temperature are thus fully re- 

 versible. 



Figure 3 shows a population before ( A ) and during ( B ) the first 

 synchronous division at 28° C. The degree of synchrony is a function of 

 the cultural conditions. It is better in a 10-ml. shallow and non-shaken 

 culture than in 150-ml. or 700-ml. shaker-flasks (previous review by 



I 



Figure. 1. The solid-line curve represents the division index (cells in cy- 

 tokinetic stages of division as a fraction of all cells counted). The broken 

 curve represents cell counts. The standard experimental procedure was a 

 shift in temperature every 30 minutes between 28° C. and 34° C, with eight 

 exposures to the high temperature. The medium was 2 per cent proteose- 

 peptone plus 0.1 per cent liver fraction L (medium B). (From Zeuthen and 

 Scherbaum, 1954.) 



