Synchronous divisions in mass cultures of the ciliate protozoon Tctrahymena pyriformis 



lapse of another cycle; it merges completely with the rest of the population. With 

 this same culture two more bursts of divisions, both of about 25 per cent., were 

 produced as a consequence of cooling to 7 C. for 2 hr. and 16 hr. respectively. 

 Thus, within the limits studied, the length of the cooling period is of no significance 

 for the size of the 'burst' obtained. From a later study of the highly interesting paper 

 by Popoff (1908) we have found a possible explanation. Popoff carefully measured 

 cellular and macronuclear volume in three different ciliates at different temperatures 

 of growth. Transfer from 25 C. to io° C. or 14° C. resulted in readjustment of body 

 size and of macronuclear volume. Probably after less than a cell cycle at the low 



.5 



03 



% 0-2 



o.iV 







22X- 



7°C 



1 n_ 



o o .0-^*7°° o j 



— °s-° 



x — x„» 



X-X— v _ X 



J I I 



1 1 4 1 l_j S_i 1 



0.3 



0.2 



0.1 



2 9 6 8/0/2/9/6/8 20 22 29 26 28 ¥f 96 



hours 



Figure 2. 



temperature both increased about 2 times in size, although the macronucleus rela- 

 tively more than the whole cell. No return was made to 25 C. Had it been it might 

 have resulted in readjustment to the smaller cell size, and thus to increased division 

 activity. Furthermore, this activity could be expected to be independent of the length 

 of stay in the cold if only the cells had been exposed long enough to assume the large 

 size typical of low temperature. 



TEMPERATURE SHIFTS ABOVE OPTIMUM 



In Tetrahymena we have obtained the best results by shifting the temperature between 

 optimum (or, in early experiments sub-optimum) and temperatures 3-5° C. above 

 optimum. In our first experiment (transfer from 24° C. to 32 C. for 3! hr. with 

 subsequent return to constant 24° C.) we obtained the formation of a group corres- 

 ponding to 34 per cent, of the whole population, and the appearance of a small 

 second peak on the curve supplies evidence of continued cycling at constant optimum 



145 



