ERIK ZEUTHEN AND OTTO SCHERBAUM 



in Tetrahymena, with rate of respiration used as a measure of mass. The paper by 

 Chalkley ( 1 93 1 ) demonstrates that in Amoeba proteus (which divides by mitosis) the 

 percentage rate of synthesis decreases as the cell approaches division. Therefore, 



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7 8 9 10 11 hours 



Figure 1 . Rate of respiration plotted on an equidistant scale against hours. 

 Of each pair of vertical lines intersecting the curves, the first one indicates 

 onset of division, and the second indicates that the daughter cells have separated. 

 All experiments start with 1 cell; the number of cells recovered at the end of 

 the experiments is indicated. In experiment 3 the log phase is over after 

 division 3, in experiment 4 after division 2. 



with division, the rate of synthesis must be stepped up, more or less as in Tetrahymena. 

 However, these data are not good enough for demonstrating a possible block to 

 synthesis prior to division. 



TEMPERATURE SHIFTS BELOW OPTIMUM 



First the effect of temperature shifts from optimum or sub-optimum to a low tem- 

 perature (7 C.) were investigated, with the results indicated in Figure 2. Multi- 

 plication continues at the low temperature. Throe different experiments are put 

 together. The first (to the left) shows the scatter of the division index in a normal 

 culture kept at 29 C. The second indicates that the division index tends to drop 

 slightly and continuously (perhaps with a lag) after transfer to the low temperature. 

 This only tells us that in the cold fewer cells enter into, than out of, division. After 

 return to the high temperature (third experiment, 15 hours at 7 C.) the culture is 

 first completely emptied of cells in division but after 1 \ hours a group representing 

 about 30 per cent, of the whole population enters into division. However, if the high 

 temperature is maintained there is no evidence of this group's reappearing after the 



144 



