EFFECTS OF LOW X-EAY DOSES 



21 



and in the controls (Table II) and similarly in the last series of Table I for 

 the rats receiving 1 r and the controls. We think that this decreased mortality 

 may be explained by a more progressive feeding of the rats by purina chow 

 during their growth. 



Not only the percentage of mortality at the 30th day after 500 r but also 

 the incidence of mortality after 500 r is the same in the conditioned animals 

 and in the controls. Indeed the mortality curves (which are not presented here) 

 are similar. 



20rats C 7days C I4days C2ldoys C 28days C42days C56days 



Fig. 1. 



These results are not suggestive of any kind of radioresistance in our rats 

 previously irradiated with low doses. We already know (Maisin, 1961) that 

 higher doses of 5, 10, 50 or 400 r administered between 7 and 42 days before 

 do not induce radioresistance in our rats however large the X-ray dose: 

 LDjQ(30), LDgojgo) I^I^90(30)- Can it be reasoned from these residts that rats 

 react differently from mice against conditioning or prior irradiation? It is 

 perhaps very dangerous to think so. We are more inclined to believe that the 

 differences obtained by certain authors are due to the fact that the uncon- 

 ditioned animals were not always subjected to the experiments on the same 

 day as the conditioned ones (Betz, 1950; Paterson et al., 1952), or that some 

 authors came to too rapid a conclusion from their data (cf. Dacquisto (1959), 

 in whose best experiment the xl is < 0-2). 



In view of these results, it was interesting to investigate the cellular 

 behaviour of the marrow. We chose the marrow because all our animals 



