POSSIBLE LINES OF INVESTIGATION IN THE FUTURE 405 



Gray and his associates (23, 24, 25, 26) have several lines of evidence 

 that cell death in the broad bean (Vicia faha) is related to the produc- 

 tion of chromosome abnormalities and not to inhibition of cell division 

 (at 3 hr). Certain of the chromosome aberrations produced by x-rays 

 and gamma radiation are likely to recover in time. Some of the effects 

 are irreversible and are probably produced by ionizations from several 

 particles acting near the same region. The effectiveness (in producing 

 chromosome damage) of gamma radiation, neutron radiation, and alpha 

 radiation was found to be in the order, 1:9:9. The x-ray yield of the 

 more permanent changes was found to be proportional to the square of 

 the dose and was affected by changes in the intensity of the radiation. 

 The majority of the chromosomal exchanges resulting from alpha or 

 neutron irradiation were between pairs of breaks produced simultane- 

 ously by the same ionizing particle. The yield was proportional to the 

 dose and was independent of the dose rate. The results of Catcheside 

 and Lea (4, 43) in a study of irradiated microspores of Tradescantia 

 demonstrate these findings and support the conclusions of Gray et al. 



Possible Lines of Investigation in the Future 



Although we are far from having a clear picture of the sequence of 

 events taking place in a mammal exposed to x-radiation, techniques 

 have been developed and progress has been made. It would be well 

 worth while to repeat many of these studies with parallel experiments, 

 using neutrons. The results not only may give evidence as to the cause 

 of greater effectiveness of neutrons but may yield information concerning 

 the fundamental effects of ionizing radiation in general. 



The LDso for gamma rays and for x-rays has been determined for a 

 number of animals. It would be well to extend these studies to include 

 the LDso for neutrons as well. Care should be taken that the animals 

 are comparable as to age, size, homogeneity, etc., in the x-ray, neutron, 

 and control groups. Care should also be exercised that different radi- 

 ations are used so as to give similar conditions of intensity and penetra- 

 bility. Table 2 shows the many gaps in our information concerning 

 the r/n ratio for dose necessary to kill half the irradiated animals. Dif- 

 ferent kinds of mammals vary tremendously in size, in relative size of 

 different organs, in metabolic rate, and in life span. A comparison of 

 the r/n ratio of the LD50 for the different kinds of mammals may help 

 to determine whether any of the following are important factors: body 

 size, metabolic rate, sensitivity to toxic agents, long life span, trauma. 



It would also be of interest to determine more definitely whether addi- 

 tivity between neutrons and x-rays is complete under some conditions 



