12 ROBERTS RUGH 



such thing as a low dose, every exposure is too high. In our times this may 

 be impractical and unrealistic, though still a concept to be respected. 



In Copenhagen, Dr. Hammer- Jacobson (1959) states: "Fetal doses of 

 less than about 1 r are presumed to cause no noticeable injury. . . . Fetal 

 doses between 1 r and about 10 r are assumed in some instances to cause 

 injuries in the form of diseases, malformations, slow development, or reduced 

 resistance, especially when the irradiation occurs between the 2nd and the 

 6th week. ... If there are additional indications, therapeutic abortion 

 should be assumed advisable. . . . Fetal doses above about 10 r are assumed 

 to involve a rather great probability of fetal injury. In such cases induction 

 of abortion should therefore be the general rule." (See Table II.) 



In all of this, reference is made to the somatic efTects, but certainly the 

 geneticist would concur. A whole body exposure of as little as 12 r will cause 

 some lymphopenia in the adult; and Brues (1959) refers to 25 r as a "low 

 dose" for somatic effects, probably because there are always at least some 

 hematologic changes. Thus, a dose of 5 r may not have measurable conse- 

 quences for the somatic tissues of the adult, but would be seriously damaging 

 to the embryo or to its progeny through effects on the gametes. 



The embryo or fetus is not simply a miniature of the adult and must be 

 regarded as a dynamic, tirelessly changing mosaic of differentiating areas all 

 integrated into an over-all pattern under organismic influences which appear 

 themselves to be immune to ionizing radiations. As long as there are undam- 

 aged building units for development which are adequate in number and 

 basically intact, these influences will attempt to organize them into a topo- 

 graphically normal, balanced embryo. But the undamaged cells cannot 

 replace those that were killed by irradiation. Any stimulus to excess cell pro- 

 duction is cancerogenic, so that the embryo may be topographically well 



Figs. 13-20 are on pages 10 and 11. 



Fig. 13. This shows 3 normal mouse blastulae suspended within the uterus at 3.5 

 days. 



Fig. 14. This shows the normal mouse embryos at the moment of implantation at 

 4.5 days. 



Figs. 15 and 16. Mouse embryos at 4.5 days (time of implantation), but following 

 x-irradiation with 15 r at 1.5 days. They show pyknotic nuclei, discarded (necrotic) 

 cells within the blastocoel, and failure at implantation. These embryos might survive 

 to give rise to deficient fetuses. 



Fig. 17. This embryo was likewise exposed to 15 r at 1.5 days and exhibits a giant 

 ceil with prominent chromosomes at 4.5 days. This is a common irradiation sequela. 



Fig. 18. This is a mouse embryo treated as that of Fig. 17, showing a prominent 

 cell with vacuolization. This is a frequent irradiation consequence. 



Figs. 19 and 20. These are members of litters dissected at 18.5 days showing 

 stunting, anencephaiy, and exencephaly, while other members of the litter appear 

 superficially to be normal. When compared with controls they, too, are shown to be 

 stunted. 



