STIMULATING EFFECTS OF CROWDING 177 



sion rate should appear after the first division in a limited volume, 

 due to the liberation of an autocatalyst from the nucleus to the 

 pericellular medium at the time of division. 



"Another attempt to accelerate division rate by the use of condi- 

 tioned medium was made with cultures of large volumes. Cultures of 

 20 drops in which an animal had lived and divided were inoculated 

 with isolated unwashed animals and their division compared with 

 that of other comparable animals also unwashed, which were isolated 

 to 20 drops each of fresh bacterized medium. 



"Animals isolated into the 20 drops in which one individual had 

 divided and lived (d) and animals isolated into 20 drops of fresh 

 bacterized medium (n.d.) at the end of 18 hours gave the following 

 numbers respectively. 



d 4,2,4,4,2,2,3,3,2,3,2,2,2,2,2,2—41 



n.d I, I, I, 2, 1, I, I, I, I, I, I, I, 2, 1, 1, 2 — 19 



"Thus indications are that an animal living and dividing in 20 drops 

 of medium conditioned it in such a way that an acceleration in the 

 division rate of other individuals isolated into it is marked. 



"Apparently in the small volumes, one individual is able to condi- 

 tion the medium sufficiently to cause rapid multiplication ; and there- 

 fore when one animal is transferred to a small volume of conditioned 

 medium, its rate of division is not faster than that of a similar in- 

 dividual transferred to unconditioned medium. But in large vol- 

 umes, one individual is not able to condition the medium immediate- 

 ly; and when, therefore, one animal is introduced into a large volume 

 of conditioned medium, division takes place faster than in the case 

 of a similar individual introduced into unconditioned medium. 



"Greenleaf and Myers in work with conditioned medium used 

 only small volumes, 2-5 drops. Both report a depressed division 

 rate in the conditioned medium, which they interpret as due to the 

 increased toxicity of the old conditioned medium." 



The importance of Petersen's work lies in the fact that for the 

 first time in the development of this phase of the problem of the 

 effect of initial numbers on the rate of reproduction in relatively 

 small subcultures, she has been able to obtain both positive and 



