428 GEO. T. HARGITT AND WALTER W. FRAY 



After a few days the cultures inoculated from the air and from 

 hay showed the presence of many chromogenic bacteria. Most 

 such were yellow but some pink and some red forms were pres- 

 ent. The culture inoculated from water showed only a few of 

 these chromogenic bacteria, but they contained forms which 

 imparted a green color to the agar of the plates. This form 

 (Bacillus fluorescens) was also recovered from the hay-infected 

 culture. But very few colonies of a lobose or running type were 

 present on the plates made at this first analysis, a few stellate 

 colonies coming from the hay inoculation. At the end of two 

 weeks the chromogenic colonies were disappearing and white 

 amoeboid colonies were taking their place, Bacillus fluorescens 

 still being abundant. When the analysis was made four months 

 later the chromogenic forms had almost entirely disappeared 

 and many of the colonies were of an amoeboid or arborescent type, 

 this being especially true of infusions inoculated through air 

 and hay. 



These analyses demonstrate a distinct succession of bacterial 

 life in a hay infusion that is left undisturbed for a long time. It 

 is quite probable that certain of these bacteria, especially those 

 which appear later in the sequence, are not favorable for the 

 continued vigor and growth of Paramecium. Such forms are 

 seen to gain the ascendency in undisturbed cultures, and this 

 ascendency is prevented, more or less, by the frequent addition 

 of fresh hay and water. 



In selecting the colonies on the plates for the production of 

 pure cultures, it was obviously out of the question to test all 

 and still keep the investigation within reasonable bounds. But 

 such a program is unnecessary, since there are certain types of 

 bacteria which are predominant and it is these which have con- 

 trol over the future of the infusion and which must therefore, 

 furnish the greater part of the food for the protozoa. To attempt 

 to get the minor organisms present would have proved mis- 

 leading in interpreting later results. Particular attention was 

 therefore paid to securing all the predominant types of bacteria 

 in pure cultures. 



