NEUTRON EFFECTS ON ANIMALS 11 



biological results of physical as well as chemical agents may be explained 

 upon the assumption that individual cells differ in the reactions and in the 

 changes produced. The weakest (most radiosensitive) succumb first, 

 then the less weak and the strongest (most radioresistant) the last of all. 



The target theory cannot be made to fit all types of biologic response 

 since by definition it made no allowance for adaptability in living organ- 

 isms. The cell (and to speak of the cell must be to include the cell system 

 with its environment) is not inert until it is dead and it is capable of re- 

 covery if it is not hurt too much: so long as it is alive it is capable of recov- 

 ery. The types of response to radiation must be learned from observation 

 under different biological conditions and the same cell may differ in its 

 susceptibility to radiation under different conditions, as for example, dry- 

 ness, metabolic activity, stage of growth and its age. There is a danger in 

 attempting too much simplification by physical explanations in such a 

 complex biological matter with its delicately poised and elaborate mechan- 

 ism of the cell system. 



The effect upon chromosomes in production of mutations in radiosensitive 

 organisms, as drosophila, is an argument against the target theory, since 

 not all cells are equally susceptible to mutational effects of radiation but are 

 also affected by temperature, nutrition, anesthesia and degree of germina- 

 tion. Environmental conditions alter the degree of response. 



The number of chromosome breaks produced are proportional to the 

 amount of the dose but independent of intensity (7, 8, 9), but neutrons are 

 more efficient in producing breaks than are X-rays. Mutation effects are 

 dependent upon the wave length, independent of intensity, but general 

 biological effects vary with alteration of the intensity and the wavelength. 

 These observations are explained on the hypothesis that a chromosome is 

 broken by the passage through it of a single ionizing particle, but that it is 

 necessary for the ionizing particle to be sufficiently densely ionizing for 

 several ionizations to be produced near the chromosome. But this still 

 gives no explanation as to what processes or chemical reactions are produced 

 to cause the break in the chromosomes. 



Radiation has a marked effect in interfering with cell proliferation and 

 the dose which produces the first recognizable changes in cell proliferation 

 is always small relative to the direct lethal dose for the same tissue. Dur- 

 ing development radiosensitivitj'- decreases as the age increases, but the 

 decrease is not necessarily progressive throughout development. It is a 

 mistake to consider all cells in the body as of the same age. The frequently 

 renewed white blood cells, for example, are young cells without much differ- 

 entiation and differentiation reduces radiosensitivity 



Apart from a direct lethal effect, cells may be so injured by radiation 

 as to be incapable of successful division and maj^ thus either perish in at- 



