Synchronous divisions in mass cultures of the ciliate protozoon Tetrahymena pyriformis 



to heat follows the same pattern; cells prevented from dividing enter division with a 

 delay which is much the same when measured at 22-24° C., irrespective of whether 

 the cells have been exposed to low or to high temperatures (cf. Figures 2, 3, 4 and 5). 

 The recovery time (90 ^ 10 min.) at 28-29° C. is three times as long as the interval 

 between the individual heat shocks in our standard treatment. That is why cells never 

 divide during treatment. However, they do synthesize, which they would not have 

 done, or would have done exceedingly slowly, at continuous high temperature. This 

 suggests that a high rate of synthesis is resumed very quickly after return from high 

 to optimum temperature. Thus, the success of heat treatment seems to depend on 

 this different rate of recovery of independent mechanisms for growth and for control 

 of the entrance, first into pre-division, then into division. 



One more observation needs comment: even though apparently cold and heat 

 block the cell cycle at some point prior to division, after return to constant optimum 

 or room temperature the cells are in a stage which in time is long separated from the 

 next division. This time is only slightly shorter than the duration of the cell cycle in 

 the heat-treated cells (cf. Figure 5). Thus heat treatment, and perhaps also cold, 

 may strike cells in a chemical situation typical of pre-division and leave them in a 

 situation more typical of post-division. One reasonable suggestion is that intermittent 

 heat treatment keeps cells in a chemical condition resembling that in a newly divided 

 cell in which we may imagine that 'precursors of division' are minimal and the capa- 

 city for growth is high. Another possibility is that during heat treatment the cells 

 pass through several incomplete cycles, by-passing the division stage. The latter 

 situation would resemble the one produced by colchicin and radiation in mitotic 

 cells. 



A problem with which we are confronted and which is open for research with 

 synchronized cultures is this: how do cells which have accumulated in themselves 

 enough material for about 4 cells behave when an artificial block to division is 

 suddenly removed? First of all, with the first division they do not spring directly into 

 4 cells, although very occasionally this has been observed. They divide in an orderly 

 manner, first into 2, then into 4, and since synthesis occurs, divisions at the high 

 after-treatment rate continue still further before normal cell size is re-established. 

 The rate of synthesis after the end of treatment continues to be lower (about 1/2) 

 than normal until normal cell size has become re-established. Thus, in Tetrahymena, 

 we observe competition between synthesis and cell division. The fertilized egg re- 

 presents an extreme case of such competition. In the ovary there is no division, only 

 synthesis; however, fertilization removes a block to cell division, and divisions then 

 follow each other in extremely rapid succession, while synthesis is completely sup- 

 pressed during cleavage (cf. discussion by Zeuthen (1953b) and by Hoff-Jorgensen 

 in this symposium). 



Thus, in this competitive interaction between tendencies for synthesis and for 

 division we find the giant Tetrahymena cells produced in heat treatment to be inter- 

 mediate between dividing eggs and log-phase cells. If we include also the ovarian, 

 rapidly synthesizing, non-dividing egg we have a series which represents all situa- 

 tions from complete ascendancy of the tendency to divide to absolute dominance of 

 the synthetic machinery. In its normal cell cycle every growing and dividing cell 

 switches to and fro between these two extremes. 



153 



