22 L. H. GRAY 



Platzman and Franck (1958) have pointed out. there also arises simul- 

 taneously in a polar medium around the molecules, which are ionized, 

 electric fields of sufticient intensity to break many — perhaps 20 — 

 hydrogen l)onds in the immediate neighbourhood. 



It is clear, therefore, that from the instant of irradiation thousands 

 of different reaction chains are proceeding simultaneously. 



Our concern is to associate ])articidar reaction chains with particular 

 forms of radiation response in the cell or in the organism. It is unlikely 

 that any generalization will be found to cover all forms of response, as 

 two examples wall suffice to show. 



When ionizuig radiation provokes the sensation of vision (Lipetz, 

 1960) the chain is evidently a short one. The response occurs in a small 

 fraction of a second, and the reaction chain is probably confined to 

 those changes initiated in the immediate vicinity of the light-sensitive 

 elements of the retina, or to the direct excitation of these elements. 

 Similarly, changes in the swimming habits of DapJmia magna, resulting 

 from exposure to a brief pulse of X-radiation, are traceable to dis- 

 turbances in the nauplius eye (Baylor and Smith, 1958), which in turn 

 may consist of a radiation-induced reduction of certain pigment mole- 

 cules, since the effects of the radiation are mimicked Ijy reducing dyes 

 and opposite to those produced by dyes having a redox potential 

 greater than -f 0-062 (relative to the hydrogen electrode). 



The retina proljably constitutes an almost ideal radiation detector, 

 since the stimulation of a small number out of a very large array of 

 identical molecules leads to a recognizable biological response. 



At the other extreme, the chain of events leading to loss of prolifera- 

 tive capacity in bacteria, in higher plants and in mammalian cells is a 

 long one, and this is the subject of this paper. 



CONSIDERATION OF DOSE-RESPONSE RELATIONS AS A GUIDE TO 

 THE ANALYSIS OF THE REACTION CHAINS WHICH MAY BE IDENTI- 

 FIED BY PHYSICAL AND CHEMICAL MEANS 



When allowance is made for experimental error, the analysis of dose- 

 response relations cannot rigorously establish the number of initiating 

 events. Zimmer (1960) has, for example, shown that it is possible to 

 devise a model with a particular distribution of target size among a 

 population of organisms, (mathematically related to the multiplicity 

 of the events needed to inactivate each target), which would lead to a 

 dose-survival curve which is indistinguishable, within the limits of 

 experimental error, from an exponential curve. It is important to 

 recognize this but, as Bertrand Russell has remarked, "It is a peculiar 



