44 L. M. ClRAY 



which gives a satisfactory fit to the experimental observations. The reUance 

 which may be placed on this expression is detei'mined by the accuracy of the fit 

 and the range of dose over which the fit is maintained — as discussed fully by 

 Zimmer ("Studien zur quantitativen strahlenbiologie"", Steiner Verlag, Wies- 

 baden, 1960) and also by Dittrich ("Trefermischkurven"' Z. NaturJ. 15b, 261, 

 1960). As explained by Lea, a given dose-response relation may, in jDrincijile, be 

 interjDreted either as an expression of the number of ionizing particles concerned 

 in the initiation of the injury, or as representing a sensiti\ity distribution of the 

 cell population to equal doses. When the form of the dose-response curve is simple, 

 embodying a linear relation between the logarithm of the survivors and dose and 

 a small extrapolation number (intercept between this line and the zero dose axis), 

 I have no hesitation in choosing the former interpretation and inferring a low 

 multiplicity of initiating events. To intei'pret a relation such as that which Dr. 

 Dewey has observed for the proportion of survivors among an irradiated pojDu- 

 lation o{ Serratia marcescens (shown as a lecture slide but unpublished), which is 

 linear with dose over 9 powers of 10, in any other way seems to l^e luu-easonable. 



From the slope of the linear portion of the dose-response curve we obtain 

 directly an estimate of the chance that at least one of the i3articles set in motion 

 when a cell is exposed to unit dose of radiation will initiate the chain of events 

 which lead to the biological effect under consideration. By the methods described 

 in detail by Lea, and with the limitations pointed out by Lea, a knowledge of the 

 slopes of the dose-response curves for different types of ionizing radiation can 

 yield approximate information as to the size and shape of the critical organelle. 



The purpose of a theory is to jDrovide a basis for fiu'ther experiments. If, having 

 exercised due caution in deriving the best dose-response relation from the experi- 

 mental data, we refuse to make the most obvious inference, we throw away a 

 valuable means of deciding which, aniong the multitude of conceivable mechan- 

 isms, are the ones most worthy of fiu'ther investigation. 



