464 JOHN W. G O W E N 



tions or "hits." For a second unit of X rays the percentage receiving 

 the point-heat absorptions would be identical with that of the first 

 unit. However, this percentage would be distributed over organisms 

 that had already received one such point-heat absorption and there- 

 fore were already incapable of surviving, and over a second group 

 that up to that time had escaped such hits. The observed percentage 

 of deaths would be less for the second unit than for the first unit of 

 X rays. For a third unit of X rays on the treated organisms there 

 would again be the same percentage number receiving a point-heat 

 absorption as for the initial or second group. Again, however, these 

 point-heat absorptions would be distributed at random over the 

 organisms that had received such absorptions from the first or second 

 units of X rays and so could not show the effects of absorption from 

 the third dose of X rays. Only organisms that had thus far escaped 

 receiving a "hit" could affect the subsequent death rate of the bac- 

 teria. The number dying on the third dose would be, again, less 

 than that of the second dose and still less than that of the first dose. 

 This would lead to a series of survivals having the general form of 

 Figure 5. These results may be expressed generally by the successive 

 terms of the Poisson distributions, where we can calculate the proba- 

 liility for receiving any of these point-heat absorptions and therefore 

 the chance that the bacteria survive any dosage. The general form 

 of these curves for actual data is as shown in the figure. 



The interpretation of the effects of X rays as terms in a Poisson 

 series is frequently used toda.y. The point-heat concept, however, 

 has been radically modified. 



2. The Sensitive Volume Concept 



A second development came in the work of Crowther {10,11). 

 He showed that a large number of X-ray quanta must be absorbed 

 by a protozoan in order to produce death. This led to the concept 

 that the protozoa were composed of volumes vital to their functioning 

 and volumes that could be destroyed without seriously injuring the 

 organisms. The size of these vital structures was much less than the 

 size of the organism. The organism was visualized as having a sensi- 

 tive spot within it. This concept did not modify the Poisson inter- 

 pretation put on the survival curve. But the observed data did add 

 to the complexity of the interpretation, for the protozoa did not die 

 according to the simple one absorption type of Poisson series. 40 



