20 H. MAISIN 



In a second series of experiments we just gave the conditioning dose and 

 on the day normally devoted to the administration of the sublethal dose we 

 sacrificed the rats to count the number of nucleated cells per mg of bone- 

 marrow and to evaluate their erythropoiesis by ^^Fe incorporation into the 

 red cells. We used male rats of the same age and weight, and counted the 

 number of marrow cells per mg in the femur using a method already 

 published (Maisin, 1959). A tibia, marrow-smear from each rat was also made 

 but we have not yet examined all of them and thus, it is too early to report 

 the results. To evaluate the erythropoiesis, we injected subcutaneously, 3 

 days before, | ju-c of ^^Fe with a specific activity of 4-5 c/g Fe, as ferric 

 citrate in isotonic, neutral and sterile solutions. The blood was withdrawn 

 from the inferior vena cava and the haematocrit recorded by a micro-method. 

 The activity in the red cells was counted in a vial scintillation counter per ml, 

 per minute and per 100,000 counts of ^^Fe injected. 



We choose to kill the annuals 72 hours after injection of the iron because 

 at this time, in our control non-irradiated rats, the activity in the red cells has 

 reached a plateau (Maisin, 1959). 



Before killing the rats we also counted the number of red and white 

 cells per mm^ and the number of reticulocytes per 1,000 red ceUs in the 

 peripheral blood of the tail. The reticulocyted were stained by the method of 

 Heilmeyer. 



We killed 6 rats in succession, 14, 28, 42, 56, 84, and 98 days after they 

 had received 1 r; after 0-1 r we killed them after an interval of 14, 28, 42 and 

 56 days. Each time, we sacrificed 6 non-irradiated controls of the same age, 

 maintamed in the same conditions from the beginning of the experiment. 



KESULTS AND DISCUSSION 



First of aU the administration of the conditioning or prior dose of 1 r or 

 0-1 r did not have any influence on the weight of the animals up to the day of 

 administration of the LDgo^gg^. 



Mortality 



The conditioning dose did not have any influence on the mortality of the 

 rats before the administration of the second dose; indeed, there were very few 

 deaths: one in a group receiving 1 r (Table I), one in two groups receiving O-I 

 r and two in a control group (Table II). 



The conditioning dose did not have more eff'ect on the behaviour of our 

 rats against 500 r, however long the length of time between the two doses 

 (Table I and II, Fig. 1); indeed on one occasion the conditioned animals died 

 sooner and on another the controls, but the difference between the two groups 

 was never statistically significant. More rats survived in the series of 0-1 r 



