10 A. C. R. Dean and Sir Cyril Hinshelwood 



of a non-mutant (and it will never differ from it by more than 

 a small factor) then an increase of mass of x per cent will only 

 occur as a result of an increase in number of approximately 

 the same amount. 



The proportion of mutants initially present is usually 

 assumed to be about 10"^ (to account for observed delays in 

 growth), and the approximate doubling of the mass of each 

 mutant, which would precede its division, would make an 

 unobservably minute contribution to the change in the total 

 mass of the culture. 



If, on the other hand, most of the cells in the culture de- 

 velop after a suitable lag period, there can be a substantial 

 increase in mass before any detectable increase in number 

 occurs. In the simplest case, where the lags are all equal and 

 each cell doubles in size before dividing, there will be a 

 100 per cent increase in mass before the numbers increase. 

 This limiting case would, however, not be observed since 

 some of the cells divide before others have completed their lag. 



Nevertheless, in several examples of adaptation to new 

 carbohydrate sources increases in mass of about 40 per cent 

 have been observed without any observable multiplication. 

 Since no cell is likely to grow to much more than about 

 double its original size, this result indicates that at least a 

 considerable proportion of the population is concerned in the 

 adaptive process (Baskett and Hinshelwood, 1951; Kilkenny 

 and Hinshelwood, 1951; Minis and Hinshelwood, 1953). 



Tests for the Presence of Mutant Forms in Massive 



Inocula 



A mutation rate of about 1 in 10^ is not infrequently 

 assumed for bacteria. This would mean a very small pro- 

 bability of any mutants at all in an inoculum of 10* and near 

 certainty of the presence of several in an inoculum of 10^. 

 Frequently the plating of 10 to 100 cells on a medium to 

 which adaptation is required leads to the formation of 

 colonies in 100 per cent yield, but only after a long lag. In 



