EFFECT OF EXCRETION PRODUCTS OF PARAMAECIUM 577 



that the differences in the division rate in the experiments are 

 far beyond the Hmits of error. 



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

 outlined point to the conclusion that paramaecia excrete substan- 

 ces which are toxic to themselves and that these substances are more 

 effective, as one would expect, when the organisms are confined in 

 limited volumes of culture medium. The logical method of pro- 

 cedure is to determine the influence of media known to be con- 

 taminated with the excretion products of large numbers of par- 

 amaecia. 



3. The effect of media in which rich growths of paramaecia have 

 occurred on the rate of reproduction of Paramaecium 



As a culture medium for this experiment an infusion of chopped 

 hay was made and boiled. After a thorough stirring, equal vol- 

 umes were poured into sterile flasks of a capacity of 250 cc. One 

 of these flasks was then seeded with five cc. of infusion containing 

 twenty-five P. aurelia from the 'stock' left over from the pedi- 

 gree culture, and the other flask was seeded with five cc. of the 

 same infusion, from which the paramaecia had been removed. 

 The flasks were kept plugged with cotton. There were then two 

 flasks containing precisely the same culture material and bacterial 

 flora, and the one differed from the other only in the presence of 

 paramaecia. After these infusions had stood for ten days there 

 was a heavy growth of paramaecia in one and none in the other, 

 and the media were ready for the experiments. From the four 

 lines of the pedigree culture of P. aureha two separate cultures 

 were isolated (designated A -\-P and A-P respectively), one of 

 which was bred on hay infusion from the paramaecia-free flask 

 (F —P), and the other on material from the flask inoculated with 

 paramaecia {F -\-P). It was necessary, of course, to remove the 

 paramaecia from the culture medium before it was used in the 

 experiments, and this was done daily by filtering it through filter 

 paper before it was used, and then examining it carefully under 

 the microscope, and picking out with a pipet the few animals 

 which had not been retained by the filter paper. As a precau- 



