STIMULATING EFFECTS OF CROWDING 173 



indication of a stimulating effect upon the rate of reproduction when 

 2 organisms, instead of i , were present ; and two series of experiments 

 were run to investigate this tendency. In the first series the animals 

 were washed once; in the second they were isolated unwashed. Ex- 

 aminations at 12, 18, 24, 36, and 48 hours failed to reveal any indica- 

 tion of acceleration due to the presence of 2, as compared with i 

 animal, in the original subcultures, either with the washed or with 

 the unwashed cultures. Again, contrary to the report of Robertson, 

 there was no speeding up of the division rate after the first fission. 

 Further experiments, made with small volumes after Petersen had 

 found that a dift'erent relation exists when animals are subcultured 

 by one's, by two's, and by four's in larger volumes, still gave the 

 same negative effects. With these small volumes the work of Peter- 

 sen is in agreement with that of Myers and of Cutler and Crump 

 rather than with that of Robertson. 



Because of the evidence of retardation in the early experiments, 

 Petersen thought that the volumes used might be entirely too small. 

 Acting on this lead and using 20 drops of culture medium (24 drops 

 equal i cc), Petersen transferred 1,2, and 4 Paramecia, which had 

 been washed three times, into this volume of bacterized medium, 

 with the results shown in Table IX. In these experiments there is 

 evidently an acceleration in the early division rate of the subcultures, 

 associated with an increase in the number of animals introduced. 

 Further experiments shown in Tables X and XI demonstrate that 

 this is not an accidental result. 



The question as to whether the accelerative effect is species- 

 specific was apparently answered for this particular organism by the 

 fact that in 5 sets of experiments similar to those reported above 

 which were found to be noticeably infested with a small ciliate after 

 the first 24 hours of isolation, the rate of division was nearly or quite 

 independent of the initial seeding. Similar results were obtained with 

 the only other set of contaminated cultures entering into this series 

 of experiments. 



So far, these experiments of Petersen's have shown that with 

 smaU volumes, /^ cc. or less, there is no acceleration with the intro- 

 duction of a single individual in the initial subculture, but that with 



