574 LORANDE LOSS WOODRUFF 



of Aa-5, Aa-20 and Aa-lf-O over Aa-2 is in every case greater than 

 the gain of Ad-5, Ad-20 and Ad-40 over Ad-2, (of. fig. 4). Again, 

 from a consideration of the data of comparable cultures changed 

 daily and that of cultures of equal volumes of media changed on 

 alternate days, it is found that the gain of the series changed 

 daily over those changed at fory-eight hour intervals is over 

 eight per cent in the case of two drops and slightly over six per 

 cent in the case of five, twenty, and forty drops. Consequently, 

 as one would expect, changing the medium on alternate days has 

 most influence in the smallest volume of medium (for details cf. 

 figs. 5, 6, 7 and 8). 



The results of some of the experiments performed, for control 

 and comparison, on the P. caudatum culture are plotted in fig 9, 

 and a glance at this will show that they are perfectly consistent 

 with those derived from the work on P. aurelia. A further check 

 on the work is also brought out in the first series of experiments 

 (A) plotted in fig. 9. At the end of the fifth period of this ex- 

 periment, another culture was isolated, line by line, from Ca 

 which was designated Cad, and thereafter in this the medium was 

 changed daily. The result, as shown in the diagram, was that 

 the organisms within the following eight days attained the same 

 division rate as that of the culture Cd, thus clearly indicating that 

 the variation in the division rate between Cd and Ca was effected 

 by the duration in time to which they were subjected to the 

 culture medium. 



Further, as a control to test the accuracy of the general method 

 employed in all the work, a duplicate control culture, designated 

 Ad-5dup, was carried and the number of divisions in this culture 

 at the conclusion of the work was exactly the same as Ad-5. Of 

 course it was an ' accident' that there should have been no varia- 

 tion during the long series of experiments, but this indicates 

 that such error as exists in the method used is very slight, and 



develop so fast that they exhaust their own food and produce excretion products 

 in sufficient amount to be detrimental to the paramaecia, whereas in the smaller 

 volumes of medium the animals keep the bacteria reduced so that they do not ex- 

 haust their food and so continue to multiply, providing food for the paramaecia, 

 but not sufficient excretion products to have a perceptible effect. 



