PARAMECIUM IN PURE CULTURES OF BACTERIA 423 



growth, and reproduction. A better index of the general vitaHty 

 could not be found and while fairly constant from day to day, 

 its fluctuations mark out clearly the periods of vigor and depres- 

 sion." The use of the division rate as an index has been ac- 

 cepted by all who have worked on the infusoria. In the culti- 

 vation of these animals Calkins used a hay infusion, always 

 made in the same wa}^, and from the same kinds of material. 

 This infusion was raised to the boiling point and cooled before 

 using. The composition of the medium was, therefore, fairly 

 constant, indeed results of later work suggest the possibility of 

 its being too constant. Temperature and other physical fac- 

 tors were either controlled or known, but the bacterial content 

 of the media was entirely unknown. Calkins says: ''The bac- 

 teria in the hay-infusion constitute the normal food of the Para- 

 moecidse. Of these, Bacillus subtilis, is, probably, the only one 

 left alive after the infusion is raised to the boiling point, and 

 this organism, therefore, forms the staple article of diet for 

 Paramecium in culture." It is not certain that Calkins was 

 entirely correct in hife infe-rence for, while B. subtilis is resistant 

 to water at the boiling point there are other bacteria w^hich are 

 nearly as resistant and may be unaffected by that temperature. 

 Furthermore there are numerous bacteria which are morpho- 

 logicall}^ like B. subtihs so the exact form could not be known 

 without the use of bacteriological technic. Even if the infer- 

 ence made were correct it would have no significance since air 

 infection of cultures on slides, or in flasks, might radically change 

 the bacterial flora of the media. Hence Calkins was working 

 with an unknown variable so far as food was concerned. 

 Woodruff ('05) used essentially the same methods of cultivation 

 for various infusoria, and was no more certain of the bacterial 

 food of these forms. 



Peters ('07 a and b) by careful chemical analyses of infusions 

 added considerably to our knowledge of the composition of such 

 fluids. Concerning the bacteria he said: "I have never deter- 

 mined the specific kinds of bacteria, but I have observed suf- 

 ficiently to see that practically the same forms are characteristic 

 of the same cycles of the cultures." Doubtless this means he 



