220 GEORGE ALFRED BAITSELL 



passed through 656 generations. Diagram 4 shows graphically 

 the life history of this culture. At the time the culture was 

 started the animals were dividing at an average rate of 2.5 divi- 

 sions in twenty-four hours. This rate gradually increased for 

 a time as shown in the graph until the culture reached the re- 

 markably high rate of four divisions per day. The records show 

 that an average rate of about four divisions per day was main- 

 tained for forty days. However, even though the division rate 

 was very high and the animals of the culture gave every evidence, 

 so far as could be determined, of being in an environment which 

 was suited to their needs, a decline in the rate began at the end 

 of the forty-day period mentioned above and this decline steadily 

 continued and finally resulted in the death of the culture (Sep- 

 tember 25, 1912). 



b. Culture Ph. This daily isolation culture kept on the beef 

 medium, was started January 9, 1912, and carried until May 1, 

 1913, a period of 478 days, during which time the culture passed 

 through 943 generations. Diagram 5 shows graphically the life 

 history of this culture. As will be noted in the graph, a division 

 rate averaging about 2.5 divisions in twenty-four hours was 

 maintained for the first twenty periods (200 days). The decline 

 which began at this point continued very slowly but steadily, 

 the rate finally falling below two divisions in twenty-four hours 

 and during the last two months of the life of the culture the rate 

 was never as high as one division per day when averaged for a 

 ten-day period. The culture finally died out at the 943d gen- 

 eration on May 1, 1913. 



In both of these daily isolation cultures of Pleurotricha lan- 

 ceolata, a study of the graphs reveals what could be termed 

 typical 'life cycles.' When once the decline began with few 

 exceptions it steadily continued and the maximum of each suc- 

 ceeding rhythm generally was lower than the maximum of the 

 preceding one. 



c. Culture Phtt. This mass culture kept on a hay infusion me- 

 dium in test tubes was started January 9, 1912, at the same time 

 that the two daily isolation cultures (Ph and Pb) were started 

 and has been carried continuously since then up to the present 



