DAMAGE IN AEROBIC AND ANAEROBIC SYSTEMS 23 



fact about the genesis and growth of new disciplines that too much 

 rigour too early imposed stities the imagination and stultifies inven- 

 tion". In my view, the analysis of dose-response relations is still one of 

 the most powerful means at our disposal for limiting our search — ■ 

 among the tens of thousands of reactions which are proceeding simul- 

 taneously in the irradiated cell — for those which lie in the pathway to 

 the loss of reproductive integi'ity. We may remind ourselves that in 

 genetics, the mathematical analysis of segregation frequencies pointed 

 to the physical existence of permanent structures carrying the genes in 

 a linear array before the microscopic study of Drosophila salivary gland 

 chromosomes revealed differences in band pattern between mutant 

 forms. On the basis of an analysis of loss of reproductive integrity by 

 vaccinia virus Lea and Salaman (1942) reached the conclusion that the 

 genetic material of the virus must occupy a small fraction of the total 

 volume before a nuclear body was demonstrated by electron micro- 

 scopy. Similarly, Preer's (1948) analysis of the radiation inactivation 

 curve of kappa in Paramecium yielded an estimate of its size which 

 exceeded the limit for visibility in the light microscope, and led to the 

 identification of the kappa particles. 



By the criterion of the observed dose-response curves, loss of repro- 

 ductive integrity would be judged, in almost all cases so far studied, 

 to proceed from energy dissipated in the cell, either by a single particle 

 or by, at most, a few ionizing particles. Dr. E. L. Powers and his col- 

 laborators (1957, 1961) have published some extremely well defined 

 linear relations between the log surviving fraction and the dose for 

 spores of Bacillus megaterium irradiated in the dry state. A very 

 similar relation for Serratia marcescens irradiated in aerobic suspension 

 has been observed recently by my colleague Dr. Dewey (unpublished). 

 After a small curvature near the origin, the experimental points define 

 a straight line. Dewey's observations are remarkable in showing that 

 the strictly linear relation is maintained over almost ten powers of ten. 

 The logical interpretation of the linear part of this curve is that when a 

 population of organisms is exposed to a certain increment of dose, the 

 probability of loss of reproductive integrity is the same for each organ- 

 ism and independent of the dose to which it has previously been ex- 

 posed. For small increments of dose it is also strictly proportional to 

 the dose increment. In physical terms, loss of reproductive integrity in 

 any given bacillus depends only on the energy delivered to the cell by 

 a very small number of ionizing particles. If the experimental observa- 

 tions had defined a line which passed accurately through the origin, 

 we should be able to infer that the loss of reproductive integrity was 

 strictly independent of the reaction chains initiated by all other particles 



