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ROBERT H. HUTCHISON 



and so forth. After allowing for recovery, the dishes were exam- 

 ined under a Leitz binocular dissecting microscope and the ani- 

 mals present were counted. A compound microscope was neces- 

 sary only when it was uncertain whether cilia or membranelles 

 were still beating in the case of otherwise motionless individuals. 

 The criterion used in deciding whether a given individual *was 

 dead or not was the cessation of all movement in cilia, mem- 

 branelles, and so forth. This had to be applied carefully in the 

 case of Blepharisma, where not much change in the optical 

 appearance of the protoplasm is noticeable, as it is in Parame- 

 cium. In Paramecium bursaria cytolysis invariably occurs at 

 the fatal temperature, and this breaking up of the. cell was taken 

 as the decisive point. 



In this way a number of different species were examined as 

 they happened to appear in the laboratory cultures. These cul- 

 tures were hay infusions seeded with some material from a pond. 

 Most of them, except quite old cultures, were slightly acid in 

 reaction. As a rule Paramecium caudatum developed most 

 abundantly in them and results of experiments with this species 

 are therefore stated first. A record of each experiment was made 

 as follows: 



Paramecium caudatum from a three weeks old culture, acid in reaction 



Some twelve experiments were performed upon Paramecium 

 caudatum, taken from various mixed cultures. The results are 

 summed up in table 1. Tabular results of experiments with 

 other species are given in tables 1 to 6. 



To represent these results graphically it is most convenient to 

 use the percentages. For example, from table 5 one may con- 



