4IO Pedigreed Race of Paratmecium [Mar. 



surface of medium exposed to atmosphere, food supply, excretion 

 products of bacteria or excretion products of paramsecia. It will 

 suffice to State that each of these factors was carefully weighed and 

 the conclusion was reached that the Variation in the daily division 

 rate in the different volumes of water is due to the excretion 

 products of the paramaecia themselves. 



If this conclusion is correct, the effects of the excretion prod- 

 ucts should manifest themselves more clearly in cultures in which 

 the organisms remained in the medium a longer period, than in 

 those in which the organisms remained in the medium a shorter 

 period of time. To test this a second series of experiments was 

 carried on 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. The culture in which 

 the medium was changed at forty-eight hour intervals showed that 

 the organisms in a volume of five drops divided over 5 per cent. 

 more rapidly than those in two drops; and that those in twenty 

 and forty drops divided over 9 per cent. more rapidly than those in 

 two drops. 



The results, then, of this series of cultures in which the organ- 

 isms were isolated every forty-eight hours, confirm the general 

 conclusion derived from the series isolated every twenty-four hours, 

 i. e., in general an increased volume of medium is conducive to 

 more rapid multiplication, and further it clearly shows that the 

 gain in division rate in the forty-eight hour series in five, twenty, 

 and forty drops over that in two drops is in every case greater than 

 the gain of the twenty-four hour series in five, twenty, and forty 

 drops over that in two drops. 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 forty- 

 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. 



It is clear, then, that all the data derived from the experiments 



