808 RADIATION BIOLOGY 



therefore, that we should look to some effect other than chromosome loss 

 for the cause of immediate lethal effects on cells. 



Hevesy (1945) believes that the basic effect of X rays on cells is the 

 inhibition of DNA formation. A comparison of the amounts of P 32 

 incorporated in the nucleic acid molecules of irradiated and unirradiated 

 tissues of the rat showed a significantly higher amount in the latter than 

 in the former. The percentage of DNA formed was not appreciably dif- 

 ferent in dividing and nondividing cells. In order to account for the 

 greater lethal effect in dividing than in nondividing cells — in both the 

 percentage of DNA formed is about the same — he concludes that inhibi- 

 tion of DNA formation in a fully developed, rarely dividing group of cells 

 is not critical, because on the average most of these cells will have time 

 to eliminate this disturbance before it is their time to divide. The more 

 frequently the cells of a tissue are dividing, the greater will be the number 

 of degenerate cells formed, because the cell will attempt to divide before 

 recovery has had a chance to take place. 



The diverse radiosensitivities of different kinds of cells to immediate 

 lethal effects as compared with mitotic effects are very striking. Tansley 

 et al. (1937), for example, found that 72 r of y rays, which reduced the 

 mitotic count in cells of the developing rat retina to a minimum of about 

 10 per cent of normal, produced a lethal effect on 11 per cent of the total 

 cells as determined 6 hours after treatment. In the Chortophaga neuro- 

 blast, however, the mitotic count is reduced to the same extent by as 

 little as a ninth of that dose, or 8 r of X rays, while 1250 times that dose, 

 or 10,000 r, causes virtually no neuroblast deaths within at least 8 hours 

 after treatment (Gaulden, unpublished). This is unrelated to the kinds 

 of radiation used; for no comparable difference has been detected so far in 

 the efficiencies of y rays and medium X rays in reducing mitotic activity 

 or killing cells. Apparently, some basic biological difference in the cells 

 themselves is responsible. This also demonstrates the fallacy in com- 

 paring radiosensitivities of different biological materials, when these 

 sensitivities are based on different criteria. 



Under the heading of immediate lethal effects we might also include 

 chromosome destruction or inactivation produced in Habrobracon eggs by 

 large doses of X rays (Whiting, 1948). If eggs treated with doses of 

 14,420-36,050 r in the first meiotic prophase or metaphase are laid by 

 females mated with untreated males, a small percentage of haploid 

 males will develop, which contain only the chromosome set of the male 

 parent. The maternal chromosomes are so adversely affected, presum- 

 ably by chromatin bridges which interfere with their anaphase move- 

 ment and with the subsequent movement and internal changes of the 

 female pronucleus, that they take no part in cleavage and are eventually 

 eliminated. 



