172 ANIMAL AGGREGATIONS 



results at the end of the first day's exposure clearly bear out the 

 author's conclusions that his mass experiments with Euglena offer 

 no evidence of any allelocatalytic effect. As would be expected, from 

 all of the work to date, Robertson's included, after the first few days 

 Jahn found a significant difference in rate of increase, with a more 

 rapid rate in the less dense populations, which he is inclined to 

 ascribe to the combined effect of relatively more food and relatively 

 less wastes. The effect of mass of Euglena, which is capable of 

 growing in autotropic media, may well differ from that of holozoic 

 organisms, particularly since their CO2 relations diverge so de- 

 cidedly. 



The work which throws most light on this tangled problem is that 

 of Petersen (1929). Petersen used Paramecium caudatum in her ex- 

 periments. They were grown in a boiled-hay medium which was 

 bacterized 24 hours before using. Animals were isolated by means of 

 a drawn-out pipette. Any attempted washing was done by re-isola- 

 tion. Throughout her experiments 24 drops equaled i cc. 



The usual method of comparing individuals of the same clone was 

 not followed in this work because it was found early in the investiga- 

 tion that there may be a marked difference in the division rate of 

 animals from the same clone. Even the division rate of sister-cells 

 showed a difference. Her method, as finally worked out, was to iso- 

 late 2 animals with approximately the same rate and time of divi- 

 sion. These were allowed to divide until 4, or 8, or 16 cells were pro- 

 duced, according to the needs of the experiments, at which time all 

 the descendants of one animal were subjected to one set of condi- 

 tions while all from the other were placed under the conditions with 

 which comparisons were to be made. Because of the re-isolation so 

 early in the life of the subculture, the lag phenomenon was avoided. 



Under these conditions, four sets of experiments were run, as 

 follows: (i) cell proximity in small volumes of medium; (2) variation 

 in volume of medium; (3) cell proximity in large volumes of medium; 

 and (4) conditioning of medium, both large and small volumes. 

 Petersen's results will be given in some detail because of their im- 

 portance. 



Preliminary experiments in 2 drops of medium gave some slight 



