45O LORANDE LOSS WOODRUFF. 



first pefiod of the present series is number 53. Two new races 

 were secured from diverse localities as already described and 

 subcultures from these were begun and made a part of this 

 experiment at once. These subcultures were designated M 

 and W. 



All these subcultures were supplied daily with the regular 

 varied culture medium. Sister subcultures (designated As, Os, 

 Bs and Ms) were isolated line by line from the above-mentioned 

 subcultures at the start of the work and were subjected to the 

 same varied culture medium but the amount was reduced to one 

 half that supplied to the parent subcultures and the medium was 

 changed at intervals of 48 hours instead of 24 hours. 



It has been shown in previous papers, 1 that the rate of repro- 

 duction of Paramecium is markedly influenced by the volume 

 and the freshness of the culture medium. This was found to 

 result from the accumulation of the excretion products, in view 

 of the fact that a medium which contains the excretion products 

 of a heavy growth of paramecia has a decidedly depressing effect 

 on the division rate of this organism. 



Accordingly it seemed that the excretion products of Para- 

 mecium afforded the most natural means of quickly modifying 

 the division rate in order to determine the effect of this on the 

 rhythms and endomictic periods. Obviously the experimental 

 conditions involve two variables excretion products, and their 

 effect, the lowering of the fission rate so that the specific 

 influence of one or the other cannot be determined from the data 

 given below. But that is not of importance in the present work 

 which is merely an endeavor, as already stated, to determine the 

 effect of normal environmental changes. 



A. 



This set of cultures was carried on in order to study the 

 periodicity of rhythms and endomixis in different races of 

 Paramecium aurelia when bred in a relatively large amount 



of varied culture medium supplied fresh daily. The experi- 



^ 



1 Woodruff, "The Effect of Excretion Products of Paramecium on its Rate of 

 Reproduction," Journ. Exper. Zoo/., X., 1911; Woodruff, "The Effect of Excretion 

 Products of Infusoria on the Same and on Different Species, with Special Reference 

 to the Protozoan Sequence in Infusions," Journ. Exper. Zoo/., XIV., 1913. 



