RADIATION IN PRENATAL DEVELOPMENT 913 



11. The finding that irradiation at a given stage causes changes only 

 in certain characters may be explained by either or both of the following: 

 (1) differentially distributed cellular damage; (2) selective mechanisms on 

 a higher level. The basis for (1) may be differential mitotic activity. 

 The fact that a precursor affected at any given stage responds in con- 

 sistent ways may be explained by the invariable response of cells which 

 have undergone a random primary change within them. The most 

 plausible mechanism is cell lethal action of somatic chromosomal aber- 

 rations which lead to aneuploidy after mitosis. 



12. Hypoxia protects markedly against radiation-induced abnormal- 

 ities to an approximately equal extent in several characters. Protection 

 is probably on the level of the intracellular damage. 



13. Although there are several exceptions, the commonest results 

 obtained on increasing the dose are increase in incidence, increase in 

 degree, and extension of the period during which radiation will yield the 

 given abnormality. These results are as expected on the assumption 

 that a threshold proportion of cells must be affected in a primordium in 

 order to produce any abnormality and that, above this threshold, the 

 number of cells affected determines degree of abnormality. Simple sta- 

 tistical considerations show that there could be variability in result even 

 in perfectly uniform material. This is superimposed on genetic variabil- 

 ity, environmental variability, and subtle differences in developmental 

 age among embryos of a given chronological age at the time of exposure. 



14. There are certain similarities but also marked differences between 

 the action of radiation on the one hand and of other deleterious agents or 

 of genes on the other. 



15. The clinical literature includes many cases in which abnormal 

 children have been born following irradiation of the pregnant mother. 

 In applying experimental results to man, it becomes apparent that 

 critical periods for the majority of gross abnormalities occur at a time 

 which corresponds to weeks 2-6 in human gestation. During at least 

 part of this time, pregnancy may still be unsuspected. It has, therefore, 

 been recommended that, whenever possible, irradiation involving the 

 uterus in women of childbearing age should be restricted to the two weeks 

 following the menses, to preclude the possibility of fertilization having 

 taken place. Experimentally, doses as low as 25 r have been shown to 

 be effective in producing developmental changes if applied at the critical 

 time. This is within the dose range used in diagnostic fluoroscopy. 



