78 THE SOCIAL LIFE OF ANIMALS 



This he proceeded to do. He found a common 

 bacterium on which his sterile Oxytricha would grow 



NUMBERS OBTAINED IN 24- HOURS FROM THE 



ISOLATION OF OXYTRICHA INTO CONSTANT VOLUMES 



WITH DIFFERENT CONCENTRATIONS Of BACTERIA 



CONCENTRN 4X 2X X X/4 X/lO 



I 



I 



3.5 9.0 11.4 5.4 3.0 



Fig. 11. The ciliate protozoan Oxytricha reproduces 

 more rapidly with a certain limited number of bacteria 

 present than with either more or fewer. (From Johnson.) 



and reproduce faster than in the ordinary medium. 

 He made standard suspensions of these bacteria in 

 sterile salt solution, at what we may call an X con- 

 centration. The bacteria could reproduce little, if at 

 all, in the salt medium, so that he knew how much 



