164 Discussion 



Yudkin has shown. But when we are measuring transformabiHty, we 

 are measuring a property of only a few of the cells, and these few could 

 be gradually deviating from the general mode of behaviour of the popu- 

 lation and so be gradually acquiring a longer and longer true cell division 

 time, which would not show up except as a slight damping effect on the 

 overall properties of the divisions of the average cells. 



Pollock: If this is a regular change in drug resistance I would have 

 thought that Yudkin' s evidence would suggest that one should look for 

 another cause and not try to fit it in with the cell cycle. Might it not for 

 instance be due to the rate of production of an enzyme which is formed 

 at a very low rate, so that you really might have a quantal effect due to 

 only one molecule being formed at a given time ? The time it takes for a 

 cell to form one molecule would not necessarily be related to cell division, 

 but it would produce a cyclic effect if that molecule was necessary for the 

 development of drug resistance. 



Yudkin: Yes, I find it perfectly possible. The important thing here 

 is that there are cycles of adaptability to resistance. If one accepts that 

 there are these very considerable cycles of resistance, and if one accepts 

 that an ordinary culture will have cells in all stages of these cycles, then 

 clearly if one tests such a culture one will find them having what you call 

 resistance, but which I would suggest is adaptability to resistance, over 

 a whole range. It seems to us that one of the explanations, if not the 

 only explanation, of the range of resistance in a sensitive culture, ex- 

 pressed in survival curves, or distribution curves, is the fact that the cells 

 show the whole range of cycles of adaptability. 



