Discussion 25 



stimulation from the plus to the minus colonies so that there appears to 

 be a uniformity of colony type which in fact does not exist, as can be 

 shown by replating. To summarize: my own experience has been that 

 there occurs from time to time, in these cultures held in D-arabinose 

 medium, a new cell type which increases very rapidly over a period of 

 about 1-1 • 5 days, and that this overtakes the bulk of the population ; 

 but one can see these two colonies side by side with one another in these 

 platings, provided they are not plated too densely. 



Hinshelwood: We never get this division into two sharply defined types 

 unless actual growth and multiplication of numbers in the culture is 

 already well under way. With the arabinose-negative t>T)e and the 

 lactose-negative type we get at any given time a distribution of colony 

 size, or for any given size a distribution of times, which is practically a 

 Gaussian distribution. 



The essential point of our argument is that before any growth or multi- 

 plication in the liquid medium begins all or most of the cells show a 

 reduction in the actual time required to form a colony of standard size 

 on the plate. I should attribute Prof. Lederberg's large colonies to cells 

 taken from a culture in which growth in the liquid has already begun to 

 make some appreciable progress. A very careful watch on total and 

 viable counts is necessary. There are two arguments : increase in mass 

 precedes increase in number, and decrease in plate lag may precede either. 

 Plating after measurable increase in turbidity may give colonies from 

 cells which have reached a still more advanced stage of adaptation. 



Lederberg: We have done a variety of experiments. If we put e.g. 100 

 cells into a plate then we get a fairly uniform development which gives 

 colonies of quite good size over a period of a week. When these colonies 

 are replated they almost invariably give exactly the same picture, 

 though it may take another week before they come out. Occasionally 

 one finds a colony that has an actual burgeoning-out from it, and when 

 such a colony is replated it gives the plus-type which immediately grows 

 up (it takes about a day and a half to reach the same colony type). This 

 experiment can be done, for example, by making rather dense platings 

 of about 10* or 10^ cells into a plate; this gives a sort of ground glass 

 appearance, and with a binocular microscope one can see the individual 

 colonies. Undoubtedly the minus-type is capable of utilizing at a fair 

 rate either D-arabinose (which I suspect is the case) or some contaminant 

 in the preparation. I think some of the imprecision that may creep in 

 here is due to the fact that we are not jumping from a zero state to a 

 100 state, but from a 5 state to a 75 state, so to speak, as compared with 

 the rate of utilization of glucose. In such densely inoculated plates there 

 appear from time to time, over a period of a week or ten days, occasional 

 large colonies which are surrounded by a very dense halo of satellites of 

 colonies from the background. Now, if we replate the large ones, which 

 are quite rare, these give new platings which consist mostly of large 

 colonies. If we replate from the satellites immediately around them, or 

 from the background growth, we generally get a small colony develop- 

 ment. So here are cells, side by side in the same environment, which have 

 quite diverse histories; and there must occur a sudden "catastrophe" 



