430 GEO. T. HARGITT AND WALTER W. FRAY 



of different kinds of bacteria were present and the selection of 

 the forms for further study was largely an arbitrary one. Those 

 bacteria were isolated which were present in the largest numbers 

 in the hope that they were the ones causing the abnormal condi- 

 tions. This is a fairly logical method of choice since only those 

 which are very abundant can be active in producing changes in 

 the infusion. In each of the abnormal hay infusions examined 

 there was a distinctly sour or putrefactive smell at the time of 

 the examination. This odor was used later as a test in deter- 

 mining what bacteria were responsible for the production of the 

 abnormal conditions. Flasks of sterile hay infusion were inocu- 

 lated with different pure cultures of the bacteria, or in some 

 cases inoculated with bacteria of a certain type but not neces- 

 sarily of one kind. At the end of a few days, or only after several 

 weeks in some cases, some of the flasks so inoculated gave odors 

 similar to those of the original infusions. The bacteria present 

 in such flasks were later used in pure cultures for feeding Para- 

 mecium to further test their unfavorableness as food. 



Not all of the bacteria which were isolated were completely 

 studied and identified. But all which were used for experi- 

 mentation with Paramecium were carefully studied and a com- 

 plete description of their characteristics will be found in a table 

 at the end of the paper. It may be said in general that the 

 bacteria isolated from abnormal infusions are more zymogenic 

 than are those from normal infusions. 



Preliminary experiments. The paramecia which were used for 

 nutrition experiments with the pure cultures of bacteria were 

 obtained in pure strains. The necessity for having all the pro- 

 tozoa of one and the same line is so obvious as to need little 

 comment. In order that results may be comparable the para- 

 mecia must be alike so far as possible and those of the same 

 pure strain are as nearly identical as can be obtained. Hence 

 all the animals used in our experiments are descendants of a 

 single individual of Paramecium aurelia or of Paramecium cau- 

 datum. In the beginning both P. aurelia and P. caudatum ^yere 

 used but owing to the lower rate of division of the latter, and 

 to its less marked adaptability to growth in depression slides. 



