424 GEO. T. HARGITT AND WALTER W. FRAY 



could tell whether bacilli or cocci were present. His chemical 

 analysis showed some correlation between the acidity of the 

 infusion and the abundance of bacteria. He suggested a prob- 

 able sequence of bacteria (correlated, perhaps, with certain 

 sequences of chemical change) which influenced or governed the 

 protozoan sequence. From this one might infer a certain kind 

 of bacterial food was essential for particular infusoria. There 

 was, however, nothing in the investigation of Peters which 

 would estabhsh that as a fact, 



Jennings ('08) was the first to fully appreciate the importance 

 of carefully considering the bacterial flora of his cultures, though 

 he made no attempt at identification. His method was prob- 

 ably fairly efficient in introducing similar bacteria into all his 

 cultures, though without the use of bacteriological technic this 

 could not be known. Also no attempt was made to prevent 

 infection from the air. He recognized that certain 'bad con- 

 ditions' of cultures were probably due to injurious bacteria and 

 under these conditions the protozoa decreased in vigor, or even 

 died. If one attempts to account for this lack of vigor or death 

 of infusoria in cultures he must consider several possible causes. 

 It is probable that death may sometimes be due to toxic excre- 

 tions of the bacteria; there may not be sufficient bacteria to 

 furnish food for the protozoa; or the bacteria present, though 

 sufficient in number, may not be suitable for food so the pro- 

 tozoa die in the midst of apparent plenty. All of these possi- 

 bilities must be considered in dealing with cultures. 



In making the cultural conditions identical in order to com- 

 pare the division-rates, Jennings says: "To make the conditions 

 of existence the same it is not sufficient to attend merely to the 

 basic fluid; the bacteria must also be the same J' This, he insists, 

 is not a mere theoretical consideration but a practical necessity. 

 The introduction of certain organisms may lead to the death 

 of the protozoa in the culture. In order to secure this identity 

 of bacteria in all cultures Jennings washed all his paramecia in 

 two lots of fresh hay infusion, all being washed in the same fluid, 

 for the second washing. This he believed would bring into the 

 second wash fluid all the different kinds of bacteria which were 



