40 



J. F. Danielli 



prevented. Secondly, cultures of proliferating organisms are 

 more likely to conceal than to reveal any deleterious changes 

 which may occur in individual cells. This is best understood 

 by considering Fig. 1. The solid line represents the rate of 

 normal proliferation of a culture, and the line B the rate of 

 proliferation of a cell which suffers a deleterious change, at 



Number 

 of cells 



10 



XA 



10 



2 



10 - 



10 - 



M 



B 



N 



Yi 



t t+x 



-*- time (linear scale) 



Fig. 1. Diagram to illustrate the changes in the proportions of 

 normal and modified cells, A for a change favouring prolifera- 

 tion, and B for a change antagonistic to proliferation. PQR are 

 the axes for the normal cells, and LMN the axes for a cell 

 which becomes modified at the arbitrary time, t. 



an arbitrary time t. PQR are the axes for the normal cells in 

 the culture, and LMN for the modified cell and its products. 

 The diagram illustrates the point that as time increases the 

 proportion of modified cells in the culture steadily falls off. 

 As illustrated, it begins at 10 per cent: after an interval 

 (t + x) it will be 5 per cent, after (t + 2x) only 2-5 per cent, 

 etc. Thus under conditions of steady growth any cell suffering 

 a deleterious change tends to become insignificant: it is just 



