STIMULATING EFFECTS OF CROWDING 171 



Yocum (1928) reported results resembling allele catalysis from his 

 work with Oxytricha. He isolated unwashed individuals to 4 and to 

 10 drops of sterile medium when 100 drops equal a cubic centimeter. 

 After 24 hours he found that when all cases were averaged, the divi- 

 sion rate in the smaller volume exceeded that in the larger by 14 per 

 cent, and that the average in the smaller volume in each of his series 

 was greater than in the larger volume by over 10 per cent. He inter- 

 preted this as meaning that the smaller volumes reach a high con- 

 centration of an autocatalyst earlier than do the larger volumes. 



The work of Petersen (1929) chronologically belongs at this point; 

 but it will be examined in more detail shortly, after considering the 

 work of Jahn (1929), who investigated the relation between density 

 of population to the growth rate in Eiiglena sp. Euglena was chosen 

 for its ability to grow in autotropic media. Single isolations were 

 abandoned on account of excessive evaporation. Of 22 cultures 

 started with washed sister-cells isolated into 3 or into 6 drops of 

 medium, 5 cultures were counted at the end of 10 days. Three of 

 these showed larger numbers in the smaller volume, one showed the 

 reverse, and the other had about equal numbers. No data are given 

 for the intervening period. 



Thereafter mass cultures were run in which the ratio of the initial 

 volume of individuals to medium ranged from i : 383,300 to i : 3,833, - 

 000. Results are recorded as obtained by a series of dilutions of from 

 0.005 to 0.55, 0.066 to 0.66, 0.089 to 0.0894, 0.081 to 0.73, 0.14 to 

 0.555, ^i^d 0.42 to 9.5 thousands per cubic centimeter of culture 

 fluid. Initial counts were made in from i to 4 days in the different 

 series. In Series 2, 3, 5, and 6' the initial count in the more concen- 

 trated medium was made approximately 1-3 days before that in the 

 least crowded series; hence it is hard to judge initial conditions. 



Only in Series i do the recorded data show the initial count made 

 at the same period in the different experimental lines; and here the 



cultures. It may be, as Petersen suggests, that the method of recording data chosen by 

 Myers does not allow his results to be used as evidence for allelocatalysis. If that be 

 true, there are similar reasons for thinking that, in so far as the i6-drop cultures are 

 concerned, they cannot be used as evidence against this interpretation. 

 ' The time of making the initial count in Series 4 is not given. 



