868 RADIATION BIOLOGY 



litter death prior to implantation. (2) The balance of the mortality is 

 due to death of individuals within surviving pregnancies (see Fig. 13-la). 

 This accounts for about two-thirds of the total death in groups irradiated 

 on days x /i, l}4, and 23^, but by day 4^, when over-all mortality has 

 greatly decreased, death of individuals has become only a minor portion 

 of the total loss. Further analysis of death of individuals (see Fig. 

 13-16, based on pregnant females only) indicates that it may occur in 

 one of two periods: (a) Animals may die before implantation, as shown by 

 deficiency in total implants in irradiated pregnant females. This type of 

 death occurs mostly in embryos irradiated on days % and 1%, eliminating 

 respectively 3.3 and 1.9 implants per surviving pregnancy. By day 

 43^, irradiation no longer causes preimplantation death of individuals. 

 (b) Embryos may die after implantation, as shown by the deficiency in 

 living implants. Further work is needed to test the present indications 

 of survival being lowest following irradiation on day 23^. 



It can be argued that, since preimplantation death involving whole 

 litters is of approximately equal importance in all groups, the majority of 

 it may not be caused by direct radiation injury to the early embryo but, 

 instead, by some effect on the implantation processes of the mother. 

 Individual preimplantation death, on the other hand, is high only as a 

 result of irradiating precleavage or very early cleavage stages. Post- 

 implantation death is greatly increased following irradiation before 

 implantation but its exact relation to developmental stage irradiated can 

 be elucidated only by further work. 



Assuming that the probability of killing a blastomere with a given dose 

 of radiation is equal in the one-cell stage and later cleavage stages, it can 

 be calculated from the incidence of death of embryos that the group of 

 those surviving radiation on days 13^-43^ probably includes some in 

 which one to several blastomeres were killed. Since virtually all sur- 

 vivors from irradiation of cleavage stages are normal, the results, as they 

 stand, indicate a considerable degree of totipotency in the blastomeres of 

 the young mammalian embryo. 



B. THE PERIOD OF MAJOR ORGANOGENESIS 



The bulk of the work on the effects of radiation on the mammalian 

 embryo deals with the period of major organogenesis and most of the 

 more careful and extensive studies fall into this group. Table 13-2, which 

 presents a condensed summary of the literature, immediately reveals the 

 almost universal discovery of abnormalities following irradiation during 

 that period. The interest in mortality is, in general, only secondary. 

 Only in the rabbit is there a dearth of reported abnormalities, but this 

 may be due to the fact that all the rabbit work was done early in the 

 century and much of it was not critical with respect to experimental 

 procedure. 



