LIFE HISTOKY OF DIDINIUM 235 



about 40 active Didinium in the culture dish. Five of these 

 were isolated and furnished material for the third cycle. Twelve 

 were killed and the remainder were fed and left as stock. All 

 of the remaining cysts were killed for cytological study. 



The average division rate during the first twenty-four hours 

 after recovery was 3.4, and for the first 25 days it remained high 

 (2.19 per day). The encystment rate for the entire period was 

 8 per cent, while the average death rate for the period of 25 days 

 rose to 14.4 per cent. This increase in the death rate as the 

 culture series grows older, and already indicated in the second 

 cycle, is interesting and significant. 



Cultural history of the Y series: Initial cycle 



The Y series, started with a second individual at the same 

 time as the X Series and treated in the same way, confirms the 

 results obtained with the X series. The first cycle (Nov. 1 to 

 Dec. 26) had the same general history as in the X series but with 

 a slightly more regular descending curve of the division rate 

 (chart C). The average for the first 20 days was 1.65 divisions 

 per day; for the second 20 days, 1.19 divisions per day, and 

 for the third 20 days it fell to 0.65 divisions per day. 



The curve for encystment is much more regular than that for 

 the X series and, with one exception in the 7th 5-day period, 

 shows a fairly steady increase (chart C, bottom). The death 

 rate was very low (chart O, top) and the appetite curve is simi- 

 lar to that for the X series (chart C, dotted line). 



The cycle came to an end with encystment of all culture in- 

 dividuals a few days in advance of the X series, and in the 128th 

 generation, on December 26. 



The second cycle {December 28 to February 6) 



The race was recovered from encystment December 28 from 

 stock material which had encysted on the 22nd, and a second 

 cycle was started with an initial division rate of 2.08 divisions 

 per day (chart D). The vitality was not as great as before, the 

 average division rate for the first 20 days being only 1.65 per 



