Synchronous divisions in mass cultures of the ciliate protozoon Tetrahymena pyriformis 



In Table I is given the DXA per total cell volume in the aliquots, the DXA per 

 cell, and also the DXA per unit volume of cells. It will be observed that total DXA 

 follows total cell volume rather closely throughout. This means that while the cells 

 grow bigger during heat treatment there is a more or less proportional increase in 

 the DXA content per cell. After heat treatment multiplication outweighs synthesis, but 

 the volume per cell and the DXA per cell decrease proportionately. The corresponding 

 morphological changes are indicated in Figure 8. Xormal (a) and heat-treated cells 

 (b) were compressed to a standard thickness of 10 microns and photographed through 

 the phase-contrast microscope. The outline of the cell body and of the macronucleus 

 are shown as tracings. It is observed that the relative increase in size is about the same 

 for the macronucleus as for the whole cell. It is therefore reasonable to assume that 

 DXA is accumulating in the macronucleus of the heat-treated cell just in the same 

 way as other specific cell components are accumulating in their natural loci. The 

 situation is comparable to the state of polyploidy in cells dividing by mitosis. 



RESOLUTION OF GROWTH IN THE SYNCHRONIZED CULTURES INTO STEPS 



In Figure 7 an insert demonstrates that in all probability growth in cell volume 

 increases more steeply after pre-division than before, thus confirming what is known 

 for single cells of other ciliates cf. p. 143'. Respiration in the synchronized cultures 

 follows a rhythmic pattern f Figure 9, open and full circles). As in the single un- 

 treated cells 'Figure 1), the rate of respiration increases for some time during and 

 after division, but it remains level for some time before division. Only one division 

 [the third in the experiment represented with solid circles] is not in agreement, out 

 of a total of ten divisions observed in the three experiments on synchronized 

 cultures.) 



If we compare Figures 9 and 1 it seems necessary to conclude that the simple reason 

 for the observed inverse relationship between synthesis and multiplication in the 

 synchronized cultures (Figure 7) is that in the treated cells the synthetic phase is 

 shorter than in normal cells, whereas the non-synthetic pre-division phase is un- 

 changed, or perhaps even somewhat extended. Another fact to be observed in Figure 

 9 is that there is no clear-cut tendency for the rate of synthesis to accelerate throughout 

 an experiment. This is very different from what was found in cultures started in 

 respirometers with one untreated cell. As discussed before fZeuthen, 1953) the curves 

 of Figure 1 can be interpreted as indicating that in the synthetic phase the rate of 

 growth is controlled by units rather than by the increasing amount of material 

 accumulating in each cell. In the pre-division stage the units ('synthetic centres'; 

 double but they do not at the same time permit synthesis in the whole cell to occur. 

 Synthesis is resumed only after the 'synthetic centres' have doubled, and they then 

 control synthesis at a double rate. 



Why did we not also find the same situation in the heat-treated cells which divide 

 fast after the end of treatment ? Perhaps because the big cell among other things is 

 overloaded also with 'centres' controlling the rate of synthesis; these 'centres', like 

 the rest of the cell, may increase in numbers but by no means double at even' pre- 

 division. 



ItI 



