Discussion 27 



explain why Dean and Hinshelwood failed to observe the two distinct 

 types of colony in their experiments during the course of training to 

 lactose. All these findings obviously do not explain the origin of papillae ; 

 nor do they exclude the possibility of some direct induced change in 

 individual cells, as has been claimed. I only suggest that it is much 

 simpler to assume that you have got some kind of mutation in a very 

 few cells, followed by selection. 



Hinshelwood: We don't agree with the interpretation of that curve. 

 We don't agree that if you plate out and then observe the relation be- 

 tween the time and the size of the colony you find this distinction 

 between the two types. Moreover, we are very well aware of the mutual 

 stimulation, and indeed have published a paper on it. But if you plate 

 a culture that has been lagging in a medium, and you get perhaps 30-40 

 colonies all in a short space of time, it is hardly possible for those to have 

 stimulated one another. If thej^ have been lagging in the medium for 

 24 hours, and you plate out enough cells to give a total count of 50 and 

 these all come up, e.g. within 30 hours of one another and not at widely 

 different intervals, then they do not have much chance to stimulate one 

 another. 



Pollock: I am thinking of the half- trained culture. 



Hinshelwood: Yes, that is exactly what I am talking about. 



Pollock: Would that not be 2 or 3 or 4 days, rather than 24 hours? 



Hinshelwood: It depends on how advanced the stage of training is. 



Pollock: We did not pursue the problem any further, and we did not 

 prove anything, but mutation followed by selection seems a likely 

 explanation. 



Hinshelwood: I do not see how this matter of having two juxtaposed 

 colonies, one affecting the other, is relevant to this particular question. 



Pollock: My point is that it might be very misleading when you were 

 scoring your large colonies and your small colonies. 



Dean : The colonies are too far apart on the Petri dish to stimulate one 

 another. 



Pollock: We find that you get stimulation over a very long distance. 



Hinshelwood: That is when a big colony has been formed and the small 

 colony is near it. 



Lederberg: I have seen this develop in the course of two days, e.g. with 

 D-arabinose. 



Pontecorvo: Prof. Hinshelwood has clearly stated that, of course, he 

 does not deny that there are changes in cell structures which segregate ; 

 on the other hand, no geneticist will deny that there are permanent 

 heritable changes definitely not due to changes in structural elements like 

 chromosomes. If we denied this of course we would deny somatic differ- 

 entiation in higher organisms! Now what seems to me an important 

 point to discuss is that of the relations between kinetic changes and 

 changes in cell structures during a process of phylogenetic adaptation. 

 This has been attempted at the level of higher organisms : Waddington, 

 for instance, has developed theories to show how responses which are 

 at first directly elicited by an external stimulus may be taken over and 

 made permanent by changes in structures later on. 



