912 RADIATION BIOLOGY 



of prenatal mortality. In the mouse, the incidence for a dose of 200 r is 

 43-79 per cent, depending on the stage irradiated. Death may involve 

 the whole litter, terminating pregnancy prior to implantation, or it may 

 occur in individuals in surviving pregnancies both before or shortly after 

 implantation. The relative proportions of the different types of death 

 depend on the stage irradiated. Virtually all survivors are normal. 



5. For comparable doses, irradiation during the early part of the 

 period of major organogenesis causes considerably less prenatal death 

 than does treatment during the preimplantation period and, from then 

 on, susceptibility to prenatal killing probably decreases rapidly with 

 embryonic age. On the other hand, neonatal death assumes consider- 

 able importance and, in the mouse, is at a peak following irradiation on 

 days 93^> and 10^. The LD50 at birth for prenatal irradiation varies 

 markedly with the stage irradiated, even when stages are separated by 

 only 24-hour intervals. 



6. A vast number of varied abnormalities have been reported as result- 

 ing from the irradiation of mammalian embryos during the period of 

 major organogenesis. A few critical experiments have shown that sensi- 

 tive periods for the induction of almost all abnormalities are short: with 

 200 r, many of them are restricted to one particular day, or, if they span 

 several days, one often stands out as the main sensitive one on the basis 

 of incidence. There are several cases of good agreement of different 

 authors on the critical periods for certain characters. 



7. Irradiation during the period of the fetus does not cause the striking 

 changes in newborns produced by comparable doses during the period of 

 major organogenesis. Immediate histological damage can, however, be 

 demonstrated and this leads to gross changes which become expressed 

 later in life. The most frequently reported defects are in the brain, 

 retina, gonads, lens, skin, and thymus. 



8. Proportional growth retardation leading to decreased body size can 

 be produced by irradiation at almost any stage after implantation. In 

 the mouse, the maximum reduction in birthweight for any given dose 

 follows irradiation between days 10^2 an d HJ^. Birthweight depression 

 for treatment on day 11 L 2 is directly proportional to dose, averaging 

 0.22 g per 100 r. 



9. Claims of upset sex ratio following prenatal irradiation and of 

 differential susceptibility of the sexes to the induction of abnormalities 

 cannot be considered proved, although it is conceivable that such effects 

 will be found. The same is true of claims that prenatal irradiation pro- 

 longs the gestation period. 



10. Although high doses to the mother may indirectly affect the viabil- 

 ity of the conceptus, there seems little doubt that most of the major 

 embryonic abnormalities are due to the action of radiation directly on 

 the embryo. 



