Biological Effects of Penetrating Radiations 101 



Exposure to X-rays or gamma rays has pronounced effects on 

 the embryological development of all species of animals which 

 have been investigated. In general, sensitivity during develop- 

 ment decreases as the age of an individual increases. This, so 

 far as the direct effect of radiation is concerned, is probably 

 associated with, although not wholly explained by, cell multipli- 

 cation and growth rate. A determination of all the factors in- 

 volved is one of the central problems of radiation.^*' ^^' ^^' ^^ 



Some light is thrown on the problem by studying the inhibitory 

 effect of radiations upon regeneration, which has demonstrated 

 a differing susceptibility of different types of cells. Or to put it 

 another way — the potencies of specific types of cells play a 

 significant part in determining the result of any given irradiation. 

 There is evidence that, under certain conditions of irradiation, 

 the process of differentiation among embryological cells is pro- 

 moted,^^' ^^^ although sensitivity to radiation is lost as differentia- 

 tion proceeds. ^^' '^^^ 



The response of the skin and its appendages to radiation has 

 perhaps been more extensively studied than in any other sys- 

 tem. ^^' ^^' ^^' ^^^ In these investigations the ultimate aim is often 

 to discover ways and means of protecting the skin from injury, 

 while permitting effective irradiation to reach the underlying 

 tissues.*^' «^ 



Observations upon the direct effects of irradiation on the gen- 

 erative system of the male rat led to one of the earliest gen- 

 eralizations on the biological effects of radiations,"* which empha- 

 sized the relative radiosensitvity of proliferating cells and the 

 relative radioresistance of differentiated cells. Subsequent ob- 

 servations have shown that this applies to all species of animals 

 investigated, though the dose level at which mitotic activity is 

 affected differs for different species. 



While such comparative studies of radiation effects on dif- 

 ferent biological material have a considerable interest, perhaps 

 more useful information is obtained by comparing the effects 

 of gradually increased doses of radiation on the same type of 

 tissue. This is perhaps most easily seen when the data are 

 arranged in tabular form (see table III). A definite gradation 



