EFFECT OF EXCRETION PRODUCTS OF PARAMAECIUM 571 



2. Effect of changing the culture medium at twenty-four and forty- 

 eight hour intervals, in each of the several volumes, on the rate 

 of reproduction of Paramaecium 



If the conclusion reached from the previously outlined experi- 

 ments is true, the effects of the excretion products should manifest 

 themselves more clearly in cultures in which the organism remained 

 in the medium a longer period, than in those in which the organ- 

 ism remained in the medium for a shorter period of time. To test 

 this a second series of experiments was carried out simultaneously 

 with those already described, and in this second series the animals 

 remained in the same medium for forty-eight hours instead of 

 twenty-four hours. There were, then, the following series of 

 cultures involved in this experiment: 



Ad 2 = P. aurelia in 2 drops of medium changed daily. 



Ad 5 =P. aurelia in 5 drops of medium changed daily. 



Ad20 =P. aurelia in 20 drops of medium changed daily. 



Ad40 =P. aurelia in 40 drops of medium changed daily. 



Cd 5 =P. caudatum in 5 drops of medium changed daily. 



Aa 2 =P. aurelia in 2 drops of medium changed on alternate days. 



Aa 5 =P. aurelia in 5 drops of medium changed on alternate days. 



Aa20 =P. aurelia in 20 drops of medium changed on alternate days. 



Aa 40 = P. aurelia in 40 drops of medium changed on alternate days. 



Ca 5 =P. caudatum in 5 drops of medium changed on alternate days. 



Each of these cultures comprised four separate lines, e.g., 

 Ad2-1, Ad2-2, Ad2-3, and Ad'^-4, and the number of divisions 

 of each of these four lines was recorded daily, at which time a 

 single organism was isolated in fresh culture medium. Conse- 

 quently the rate of division of Ad2 is the average rate of division 

 of the four lines comprising it. 



The culture in which the medium was changed at forty-eight- 

 hour intervals showed that the organisms in a volume of five 

 drops divided 5.3 per cent more rapidly than those in two drops; 

 those in twenty drops divided 9.3 per cent more rapidly than 

 those in two drops, and those in forty drops divided 9.25 per cent 

 more rapidly than those in two drops (fig. 4, B). The details 

 of each of the four experiments are graphically shown in fig. 3, 



