As L cells go through their growth-division cycle they acquire the capacity to respond progressively more strongly to certain standard changes in the temperature of the environment. Using techniques described earlier, we found that chilling to 1, 6 or 10 °C for 1 h had little effect on the timing of the forthcoming division. Conversely, heating for 1 h to temperatures between 41 and 42 °C had a strong effect. Generally, the older the cell when heated, the more extended is its generation time; in other words, the longer is the forthcoming division postponed. We found evidence that late in the cycle the cells undergo transition from a state in which they are maximally delayed with respect to the performance of a division to one in which they are less delayed.

We attempted to synchronize cell divisions with single and with series of heat shocks (41.6 °C for 1 h). Like our predecessors in the field, we obtained only partial synchrony. However, because L cells appear to prepare for division between shocks, and because heat shocks tend to reverse such preparations for division, we find reason to continue these experiments, using previous experience with Tetrahymena and Schizosaccharomyces as a guide. Both the latter cells respond to proper temperature treatment with synchronous cell division.

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