288 L. L. WOODRUFF. 



ducted. The work on the infusorian life history has clearly 

 shown that many species of infusoria, when bred on a more or 

 less constant culture medium, pass through quite definite cycles. 

 Calkins, Woodruff, and Gregory have shown also that specific 

 changes in the environment at critical times may "rejuvenate" a 

 culture and lengthen its life for long periods. It is demonstrated, 

 I believe, that the life history of the infusorian is cyclical when 

 subjected to a constant environment, and it is also demonstrated 

 that the life history may be lengthened by the timely use of 

 various stimuli. 



I have defined a cycle as " a periodic rise and fall in the fission 

 rate, extending over a varying number of rhythms, and ending in 

 the extinction of the race, unless it is 'rejuvenated* by conjuga- 

 tion or changed environment} 1 l This suggests the idea that it 

 may be possible to eliminate the cyclical character of the division 

 rate by constantly subjecting the organisms to a varied environ- 

 ment and the present investigation is devoted to this aspect of the 

 problem. In a former paper 2 I have given an outline of my 

 studies up to May, 1908, on the life history of Paramecmm when 

 subjected to a varied environment. The present paper presents 

 the data to June 29, 1909. 



II. METHODS. 



A "wild" Parameciinn aurelia (candatuui) was isolated from a 

 laboratory aquarium on May I, 1907, and placed in about five drops 

 of culture medium on an ordinary glass slide having a central 

 ground concavity. When this organism had divided twice, pro- 

 ducing four individuals, each of these were isolated on separate 

 slides to start the four lines, I-a, I-b, I-c and I-d which compose 

 this culture (Paramecium I). 3 The culture has been continued 

 by the isolation of an individual from each of these lines almost 

 daily throughout the life of the culture up to the present time 

 (June 29, 1909). A record has been kept of the daily divisions 

 of each line, and the average rate of division of the four lines of 

 the culture and this again averaged for five- ten- and thirty-day 



1 Woodruff" ('05). 



2 Woodruff ('oS 2 ). 



3 For further details in regard to technique see Woodruff ('05). 



