862 RADIATION BIOLOGY 



approximate measure. Unfortunately, the importance of controlling 

 even the postconceptional interval has eluded some investigators 

 altogether. 



For a few reasons, then, the mammal is not the most suitable material 

 for investigating the effects of radiation on development. Conversely, 

 however, radiation is an excellent tool for approaching mammalian 

 embryology from an experimental point of view. As opposed to other 

 deleterious agents (e.g., injected poisons), radiation does not have to 

 "pass the placental barrier" and it reaches the embryo in calculable quan- 

 tities. Timing of treatment is not complicated by the unknowns existing 

 for other agents (e.g., how soon effective concentrations are built up in 

 the circulation or how rapidly the poison is destroyed or eliminated by 

 the organism). Finally, since the action of radiation has a general dis- 

 tribution throughout the organism, selective response of structures may 

 be expected to indicate patterns of sensitivity intrinsic to the embryo. 



Work on the effects of radiation on mammalian prenatal development 

 will thus be reviewed from the points of view of (1) the contributions to 

 mammalian experimental embryology and developmental mechanics, 

 and (2) the human implications of the existing experimental material 

 supported by pertinent clinical findings. 



The early investigations in the field were, unfortunately, not well con- 

 trolled, not only with respect to radiation factors, as might be expected, 

 but especially as regards timing of developmental stages. They, therefore, 

 do not contribute substantially to point (1) except for scattered incidental 

 information. For this reason, they will not be discussed in detail but are, 

 for the sake of completeness, included in the summarizing tabular mate- 

 rial (Tables 13-1, 2, 7). 



II. EXPERIMENTAL FINDINGS 



Although it would be most profitable to discuss separately the results of 

 irradiating each of the conveniently landmarked stages from conception 

 to birth, only a very small number of past investigations is suitable for this 

 type of analysis, and rougher groupings must, therefore, be made. Con- 

 venient points of division can be derived from a survey of the entire 

 gestation period of the mouse (Russell, 1950) in which irradiated stages, 

 differing by 24-hour intervals, ranged from day }4 postfertilization to 

 near term. The three broad phases which emerged from this study were 

 as follows: (1) irradiation during the preimplantation period gave a high 

 incidence of prenatal death but almost no abnormalities in survivors to 

 term; (2) irradiation with the same dose during the period of major 

 organogenesis yielded a high incidence of abnormalities at birth but much 

 less prenatal mortality; and (3) irradiation during the fetal period 

 (growth, minor organogenesis) did not cause prenatal death, and no gross 



