IONIZING RADIATIONS: EFFECTS ON EMBRYO, FETUS 9 



periods have increased the intrauterine mortality by lO'^r, so that in the 

 early embryo we are probably dealing with the most radiosensitive stage in 

 ontogeny. No exencephalies ha\e appeared among thousands of unirradiated 

 control embryos or in those irradiated after completion of organogenesis. It 

 has been produced, however, by other traumatic conditions (Figs. 19, 20. 

 22.24). 



The term low dose should be defined here. Green ( 1959), the geneticist, 

 says: "There is no totally sate dose of radiation," so that to him there is no 



TABLE I 



Malformations .Among Normal Offspri.ng of CFi X CFi Mice ' 



Offspring Xumber Per cent 



Pregnancies 61 



Total embryos 630 



Normal embryos 591 94 



Dead embrvos 2 0.3 



Resorptions 37 5.7 



Exenccphal 



les 



* This table shows that among 63(1 unirradiated CFi mouse embiyo>, 

 not a single exencephaly (brain hernial developed. However, note that 

 there were almost 6% resorptions and two dead embryos. This is an 

 expected ratio and may be due to genetic causes. \t no time in our 

 experience, while examining thousands of mouse embryos. ha\e we 

 found the congenital anomaly of e.xencephaly among the control mice. 



Figs. 1-12 are on pages 6 and 7. 



Figs. 1 and 2. These are normal mouse eggs seen during the first 24 hours after 

 conception. Fig. 1 shows the egg at the moment of sperm entrance and Fig. 2 shows 

 the two pro-nuclei and the first polar body. Highest percentage of resorption follows 

 irradiation at this stage. 



Fig. 3. This shows a group of normal mouse embryos at 1.5 days in the 2-cell 

 stage. This is probably the most radiosensiti\e period with respect to the production 

 of irradiation congenital anomalies. 



Fig. 4. This is a mouse egg at 1.5 days which had recei\ed 50 r at 0.5 days and 

 shows a hyperchromatic nucleus. 



Figs. 5 to 8. These show various stages in the disintegration of the mouse embryos 

 following exposure to 15 r at 1.5 days. Note that in some cases the pro-nuclei are 

 being extruded from the cytoplasmic mass. It is unlikely that the residual cellular 

 material could survive. Howc\er. the majority of eggs exposed at this time and to 

 this level of irradiation would survive. 



Figs. 9 to 12. These are all irradiated embryos exposed to 15 r at 1.5 days and 

 examined at 2.5 days. Note pyknosis, hyperchromatism, and fragmentation of the 

 embryos. 



