i62 ANIMAL AGGREGATIONS 



multiple in excess of this. He reports that he has obtained a rate of 

 reproduction from two and a half to ten times that which might 

 otherwise be expected. In his later work Robertson found this effect 

 to be more marked when the transplants are freed from contamina- 

 tion with the parent culture medium by repeated washings. The in- 

 creased rate of reproduction, which Robertson calls the "allelocata- 

 lytic effect," does not depend upon conjugation, because this does 

 not occur within the conditions of the experiment. Robertson at- 

 tributes the stimulation to the diffusion of some agent from the 

 organisms into the culture medium, which accelerates their repro- 

 ductive rate. When more than one organism is initially present, the 

 concentration of this substance within the organism is higher; and 

 the rate of multiplication is increased as a direct consequence. 



Table VIII gives Robertson's allelocatalytic data from his 1921a 

 paper, showing the effect of placing 2 Enchelys farcimen together in 

 a single drop (0.08 cc.) of culture medium as compared with isola- 

 tions of single individuals into the same amount. The figures given 

 include all cases recorded in this paper, except those which Robert- 

 son says were run under conditions unfavorable for allelocatalysis 

 but which were run and described to test out the conditions under 

 which allelocatalysis might occur. Omitted cases include those in 

 which the parent culture was over 3 days old ; those in which one of 

 the two was purposely killed; and those introduced into bacteria- 

 free media. 



Jahn (1929) calls attention to the fact that the averages of the 

 generation time, during the first observation after isolation, in four 

 much cited experiments of Robertson's, Nos. 238A, 240A, 23 7 A, and 

 242A, show a variation of +31 per cent, about a median of 9.1 hours; 

 and that one pair of experiments, Nos. 310A and 311 A, with i iso- 

 lated individual and one culture of 2 individuals, shows a similar 

 variation of + 20 per cent. He reasons that since Robertson has 

 listed these experiments in several publications they must be typical, 

 but that the acceleration in experiment No. 31 lA is less than the 

 variation in i -animal cultures and that therefore all Robertson's 

 results must be questioned. 



However, enough data has been cited in Table VIII to show that 



