SYNCHRONIZED GROWTH IN TETRAHYMENA CELLS 155 



goes from one division to the next. As a result there is a pihng up of 

 products to be used in later division. The temperature shocks thus far 

 studied all represent changes from an optimum growth temperature 

 (28° C.) to higher and to lower temperatures. As far as the simple 

 measurement of time can tell us, all shocks tend to undo some prepara- 

 tion which the cell has previously made toward a division. Thus, it is 

 argued, the shocks push the equilibrium reactions back, thereby coun- 

 teracting the continuous piling up of products essential for division, or 

 even reducing in amounts such products present at the time when the 

 shock is initiated. A cell, when exposed to a temperature shock, there- 

 fore becomes set back in biological time which is measured on a scale 

 which goes from division to division. Figures 7, 8, and 9 are selected to 

 illustrate these points. Figure 8 shows that partial or complete adapta- 

 tion to the shock temperature may take place during an extended 

 shock: the cell is first set back, but it may again push toward division. 

 It is part of the views developed that division and temperature shocks 

 have similar eflPects, in the sense that the cell needs a long time to re- 

 cover from both events before it can divide again. During this recovery 

 it changes from a state when it cannot be set back by a temperature 

 shock, to a state when it can be maximally set back, in all cases to a 

 very early time-point of its recovery from either division or the previous 

 temperature shock. Synchrony is induced because in a mass population 

 a heat shock, or (better) a series of shocks, pushes all cells back to a 

 common biological time. In the paper this aspect is dealt with under 

 the heading "Physiological Mechanisms." 



Any suggestion concerning biochemical or biophysical mechanisms 

 in the induction of the division synchrony must take into account the 

 closely similar physiological response to cold and to heat. It must also 

 touch upon the strikingly continuous changes in the response of the 

 cells to a temperature change, both when the age of the cell is varied 

 and when the temperature of the shock is selected over a wide range. It 

 is for these reasons that the present account stresses the word "balance" 

 and avoids the term "denaturation." The analysis with amino-acid ana- 

 logues and with base analogues point toward an absolute or relative 

 deficiency of the newly synchronized cells with respect to proteins 

 essential for division. If the normal preparation for division, from di- 

 vision to division, requires that various proteins are synthesized in ac- 

 curate balances, then one could visualize that temperature shocks 

 change this balance— in a difiFerent way depending on the nature of the 

 shock, but with the common result that recovery by more protein syn- 

 thesis must take place in a constant temperature environment. 



The suggested temperature efiFects on the balance of proteins might 

 be primary e£Fects or they might be secondary to temperature effects 

 on many levels. We need not necessarily suggest thermal inactivation 



