170 ANIMAL AGGREGATIONS 



effect of the presence of 2 animals is less marked than in the smaller 

 volume." He did not attempt to carry this further to find the rela- 

 tive effect of a further increase in volume. 



Greenleaf's work is chiefly to be criticized because of his taking 

 average division rates for a period of 5 days instead of following the 

 actual division rate per day — the more so since Robertson had pre- 

 viously called attention to the necessity of such care and Peskett 

 (1925) had used particular care to examine his cultures at fairly 

 frequent intervals in his later studies upon the rate of growth in 

 yeast cells. 



In keeping with his other results, Greenleaf found that isolating 

 infusorians into 5 drops of culture medium causes a higher rate of 

 reproduction if the cells are re-isolated after 24 hours than if they are 

 allowed to stand for 48 hours in the same medium. , 



Myers (1927) worked with washed Paramecium caudatum and 

 found that ''increasing the numbers of individuals present in a given 

 volume does not increase the rate of reproduction either from the 

 beginning of the cultures or after the first fission." In his work he 

 used 2-16 drops of culture medium, measuring from o.i to 0.8 cc. 

 His observations were usually made three times per 24 hours, at 

 intervals of 6, 6, and 12 hours. 



Petersen (1929) justly comments on a part of this work of Myers, 

 showing that his system of taking observations at 6-, 6-, and 12-hour 

 intervals may mask significant results since an indicated interpreta- 

 tion of his data shows that he does not regard changes of from 6.0 

 to 8.4 hours before the first division time as being significant, al- 

 though the higher number represents an increase of 40 per cent. Of 

 course, the technique used may require greater differences before 

 significance is reached.^ 



' In the light of this criticism supported by tabular evidence, it is hard to understand 

 Jahn's (1929) diii&culty which causes him to assert that Petersen has selected certain 

 data from two sets of Myers' experiments. Actually she used all the data published by 

 Myers concerning i6-drop cultures which bore on this point, and lists all three sets of 

 these experiments both in her summarizing table of Myers' work and in the accompany- 

 ing discussion. From other remarks concerning Petersen's work, Jahn seems to have 

 overlooked the fact that she was interested in contrasting the effects obtained by 

 testing for aJlelocatalysis in small and in relatively large volumes of medium, from 

 which came her particular attention to the results reported by Myers from his i6-drop 



