PARAMECIUM IN PURE CULTURES OF BACTERIA 425 



present in the original cultures from which the paramecia were 

 taken. Finally he added a definite amount of this second wash 

 fluid to each culture in which he was growing Paramecium. By 

 this method he hoped to have the same bacteria in all his cul- 

 tures. Such a method is more cumbersome than to use exact 

 bacteriological methods, and has other serious drawbacks. Jen- 

 nings could not have any precise knowledge as to whether he 

 had accomplished his aim of securing uniformity of nutritional 

 conditions; he could only assume that he had. A more serious 

 disadvantage is the impossibility of later duplicating the experi- 

 ment exactly, and this means no other culture could fairly be 

 compared with this one on account of ignorance concerning the 

 bacteria present. 



Woodruff in 1909 introduced some modifications into the culti- 

 vation of Paramecium on depression slides by making his media 

 from different materials, hay, grass, pond weeds, material from 

 swamps, ditches and the like. He believed in this way he could 

 make conditions more nearly normal, than by the constant use 

 of a similar medium. Calkins' paramecia died out when kept on 

 a constant medium, and Woodruff beheved this was due to the 

 uniformity of the medium. Both at this time and later ('11 a) 

 in analyzing the effect of excretion products on reproduction of 

 Paramecium, Woodruff depended on the bacteria carried with 

 the animals to inoculate his culture media. He assumed the 

 bacterial content of the cultures was the same, hence believed 

 the differences observed in the division-rates were due to the 

 different volumes of culture fluid, i.e., to the amount of excre- 

 tory products of Paramecium itself. But since it is probable 

 that the bacterial flora was not uniform in all the cultures there 

 is a possibility that the excretory products of the bacteria may 

 have been the occasion for the differences in the rates of fission. 



Woodruff and Baitsell ('11 c) found a 0.025 per cent solution 

 of beef extract to be a better culture fluid than hay infusion, 

 since protozoa did as well in a constant medium of this solution 

 as in a varied hay infusion. There was, as they admitted, a 

 variation in the bacterial content in their different slide cul- 

 tures, but they believed this variation was so slight as to be 



