174 RUTH L. PHILLIPS 



will not live upon B. proteus, it would seem that Popoff' s results 

 may have been due to the effect of unfavorable bacterial food 

 rather than the result of an unfavorable medium. 



Any experiments dealing with feeding Protozoa pure cultures 

 of bacteria necessarily involve a special technique. It is, how- 

 ever, not difficult for the protozoologist to master. Hargitt and 

 Fray ('17) were the first to devise a method for rendering Para- 

 mecium bacteria-free, which is very simple and involves no new 

 method of handling the animals. All that is necessary is to have 

 absolutely sterile apparatus and media. Their method con- 

 sisted in a thorough w^ashing of the animals in several changes of 

 sterile water. The bacteriological tests for controlling cultures 

 in the progress of the w^ork are of the simplest, and with the pre- 

 pared media now on the market, take little more time than does 

 the making of ordinary hay infusions. In my experiments I 

 have so shortened the method described by Hargitt and Fray 

 that it is possible to carry a very large number of cultures, and 

 my work has shown that with care as to the sterility of all appa- 

 ratus, one need not fear contamination for some time. This 

 reduces the amount of washing necessary to a minimum — an 

 important factor in economy of time. It would seem, therefore, 

 that there should be no technical obstacles in the way of further 

 investigations concerning food, and that much of the work now 

 under way regarding the effect of glandular extracts and vita- 

 mines on the Protozoa might be made more exact by the use of 

 these methods. 



The method of making control cultures containing chance 

 mixtures of bacteria is important. Jennings ('08) describes his 

 method for keeping the mixture of bacteria constant. He took 

 Paramecia from vigorous stock cultures and washed them in a 

 large amount of culture fluid. Since the animals were from dif- 

 ferent infusions, a very representative mixture of bacteria war 

 obtained in the washing fluid. This ^vas used to seed the cul- 

 tures in which he wished to grow pure lines of Paramecium. By 

 doing this every few days he was able to keep the bacterial con- 

 tent of his cultures at an optimum efficiency as to food. It 

 seemed to me that a chance ,rnixture even more reliable than this 



