EFFECT OF EXCRETION PRODUCTS OF PARAMAECIUM 567 



The experiments were conducted simultaneously and in the same 

 place for all the volumes of medium, and therefore the temperature 

 was the same. 



It is believed that the exceedingly slight pressure variations 

 in the different volumes of water was without appreciable effect. 

 There is certainly no evidence extant that free-living protozoa 

 are sensitive to such exceedingly small changes, and therefore 

 this factor will be dismissed as not entering into the effects noted. 



The question of increased surface exposure to the atmosphere 

 in the larger volumes of media employed facilitatmg the exchange 

 of gases, as oxygen and carbon dioxide, is possibly one of more 

 importance, and accordingly care was taken to use receptacles 

 of different capacities and shapes for the different volumes in an 

 attempt to equalize the proportion of surface to volume of the 

 different amounts of medium. Obviously it is practically impos- 

 sible to make them actually equal, but certainly the precautions 

 taken were sufficient to render this factor negligible. 



The food supply is obviously a factor of great importance. 

 The culture medium consisted of an infusion of hay which was 

 raised to the boiling point to eliminate the possibility of contam- 

 inating the culture with 'wild' paramaecia, and was used after 

 it had cooled. No precaution was taken to make the infusions 

 exactly the same each time, but the same infusion was used for 

 all cultures at the same time. This was made up fresh every 

 forty-eight hours, beginning with the first day of each experiment, 

 thus, in those experiments in which the medium was changed 

 every twenty-four hours, the organisms were transferred to some 

 of the medium still remaining in the stoppered flask from the day 

 before. This flask was shaken before it was used, and there is 

 every reason to believe that the organisms in each amount of 

 medium received exactly the same food. Slight variations in 

 the bacterial flora were avoided, it is believed, by the fact that at 

 the beginning of each experiment all the paramaecia came from 

 the same environment, and consequently the bacteria transferred 

 with them when they were isolated would presumably be the 

 same in each case, and if they were not the same, or if the new in- 

 fections from the air, which must have occurred in the course of 



