132 THE ACTION OF THE LIVING CELL 



nificantly increased when two animals are present in 

 the same drop. Similarly, Calkins (1926), working 

 with Uroleptus, found the division rate to be inversely 

 proportional to the number of protozoans introduced 

 into a given volume of culture medium. 



From these experiments it seems likely that these 

 microscopic animals release a substance which ac- 

 celerates the growth of the same species. With Pare- 

 mecia, variable results have been obtained by different 

 workers (reviewed by Allee, 1931). In part, this 

 seems to be due to differences in experimental tech- 

 nique, although the recent work of Petersen (1929) 

 has thrown considerable light upon the factors which 

 influence the growth of this ciliate. 



By alterations in experimental procedure Petersen 

 has been able at will to obtain results which confirm 

 or negate Robertson's conclusions, for she has shown 

 that the volume of the medium employed is a decisive 

 factor in the final results obtained. For example, she 

 found that when the volume of media was slightly less 

 than 1 cc, the division rate of individual Paramecia 

 was markedly accelerated by the presence of other 

 animals of the same species, whereas in larger volumes 

 of fluid, the organisms were without apparent influ- 

 ence upon one another. This fact, of course, may be 

 due simply to a dilution effect, for if a given concen- 

 tration of the growth stimulant derived from a few 

 animals is necessary in order to accelerate the division 

 rate of others present, it is evident that simple dilu- 

 tion of the pericellular fluid will lower this concen- 

 tration below the necessary minimum. 



It has long been recognized that in mass cultures of 

 protozoans, some metabolites are excreted into the 



