RADIATION IN PRENATAL DEVELOPMENT 891 



Reduction in litter size at birth in the group irradiated on day 8 is com- 

 pletely accounted for when these abnormals are added to the number of 

 placental rests (but agreement for day 7 is not very good). Kaven sug- 

 gests that the various effects may be explained by embryos being in 

 slightly different stages of neural tube closure at the time of irradiation on 

 day 8. The developmentally (not necessarily chronologically) youngest 

 are most sensitive and die early (placental rests at time of observation). 

 This is supported by the finding that irradiation on the preceding day 

 (day 7) gives a greatly increased prenatal mortality. Slightly more 

 advanced embryos respond with development of pseudencephaly and die 

 shortly before term. Still more advanced embryos are at least externally 

 normal for a considerable period, then about 3 days before birth develop 

 meningocele and show reduced postnatal viability. Finally, the most 

 advanced embryos are not affected at all. 



Pagenstecher (1916) concentrated his studies on rosette formation in 

 the retina, after von Hippel and Pagenstecher had demonstrated very 

 early (1907) that various eye abnormalities could be obtained from the 

 irradiation of rabbit embryos. In four fetuses, observed 3-4 days before 

 term, which had been irradiated between days 9 and 15 postfertilization, 

 he found three with rosettes (five out of eight eyes). He believed that 

 rosettes were the simplest type of retinal damage and that each repre- 

 sented the apex of an altered fetal fold of the retina. 



Hicks (1950) irradiated rat and mouse females "in the second and third 

 week" of pregnancy (without closer timing of the stage of treatment) and 

 reports the resulting nervous system changes. Mention is made of the 

 fact that extraneural damage is not obtained with doses below 400 r. 

 Since this is in contradiction to all other work (already reviewed) on 

 mouse and rat for irradiation during the second week, it seems probable 

 that virtually all of Hicks' data came from exposure during the third 

 week. They will therefore be discussed in the section dealing with 

 irradiation during the period of the fetus. 



c. Comparison of Experiments. In an attempt to derive more general 

 conclusions about certain of the abnormalities which have been obtained 

 through irradiation of embryos, Figs. 13-4, 5, 6, 7 have been constructed 

 from data contained in eight publications of five groups of investigators 

 reviewed in preceding sections. The figures include all abnormalities 

 reported by Job et al. (1935), Kaven (1938a, b), Warkany and Schraffen- 

 berger (1947), Wilson and Karr (1951, 1950) ; and a sample of the abnor- 

 malities reported by Russell (1950, 1949). The data of Kosaka, of 

 Pagenstecher, of Hicks, of Murphy and de Renyi, and of early authors 

 contain either insufficient information or none at all on stage of irradia- 

 tion. They, therefore, had to be omitted from the comparison since they 

 can contribute little or nothing to conclusions about critical periods. 

 Also omitted are the experiments of Raynaud and Frilley which involved 



