PARAMECIUM IN PURE CULTURES OF BACTERIA 453 



made by raising the cover of the dish only sufficiently to permit 

 the insertion of the capillary portion of the sterile pipette. If 

 these precautions are followed the bacteria will remain practi- 

 cally unchanged for a considerable time, so far as the entrance 

 of new forms are concerned. 



5. The degree to which these precautions are to be observed 

 will depend upon the requirements for uniformity of conditions. 

 Paramecium seems to keep in good condition and grow best on 

 a diet of different kinds of bacteria and for most work the usual 

 methods are satisfactory. It is only where the needs of the 

 experiment are for the greatest constancy in all environmental 

 conditions that the above refinements are called for. 



6. While a mixed diet is best it must not be forgotten that the 

 number of bacteria on the hay and in the air are very great 

 (30 were obtained in pure culture by us, and these are only a 

 portion of those present), and some of these are injurious to 

 Paramecium. The food is a real factor to be considered and it 

 may be controlled as precisely as desired. For critical work 

 chance infection of media, or cross infection of cultures by the 

 transferral of several animals with a single pipette, is not a 

 satisfactory, nor a scientific method. 



