RADIATION IN PRENATAL DEVELOPMENT 881 



(22 per cent) had eye defects (about % bilateral) combined in 8 of the 

 animals with one of the other two abnormalities. t Breaking down this 

 generally sensitive period, it appeared that hydrocephalus resulted from 

 irradiation on the ninth day and jaw abnormalities on the eleventh day, 

 while most of the eye defects were produced by treatment on the tenth 

 day. Job et at. can thus be considered to have brought the first demon- 

 stration of well-defined critical periods for abnormalities in different 

 characters. Incidence for individual treatment days cannot be calcu- 

 lated since separate totals are not given, but the qualitative results are 

 included in Figs. 13-4 to 13-7 for purposes of comparison with the findings 

 of other investigators. 



In an experiment essentially similar to the above, Kaven (1938a) 

 obtained litters from over 162 mouse females irradiated on any one of 

 days 7 to 19 5 with a constant dose of 178 r (at the level of the embryo or 

 fetus). Two improvements over Job's method were (1) the use of 

 genetically homogeneous material, and (2) observations on a large control 

 group (Kaven, 1938b). Abnormals are reported for each group sepa- 

 rately and give clear indications of the existence of critical periods even 

 though percentage incidence can usually be only estimated (since Kaven, 

 like Job, does not report the number of young observed within each 

 group). The qualitative results are included in Figs. 13-4 to 13-7. The 

 most frequently affected structure, the tail (abnormal in over 130 new- 

 borns), is sensitive only between days 9 and 14, with peak sensitivity 

 probably on day 11. Brain hernias were obtained exclusively from 

 irradiation on day 8. Although both the incidence and total number in 

 this group were small in the original series, a separate experiment (Kaven, 

 1938b — see p. 890), in which irradiation was given only on days 7 or 8, 

 established that the original cases had not been spurious. Four other 

 abnormalities are reported, in the survey series, from treatment during 

 the period of organogenesis: head malformation and head hemorrhage, 

 each appearing in one of eighteen litters irradiated on day 10; hydro- 

 cephalus (apparent a few days after birth), occurring in about 10 per cent 

 of the animals irradiated on day 12 and in lower proportions from the 

 treatment of days 13 and 10; and skin defects arising later in life as a 

 result of irradiation on days 13 and 14 postconception. (Other abnormal- 

 ities produced by Kaven — e.g., torsion of limbs, digital abnormalities- 

 were apparently not noticed by him but are apparent from the photo- 



8 Kaven followed two systems of timing pregnancy: (1) by actual observation of 

 copulation, in which case his "first day" equals day %$ to 1%, depending on hour of 

 mating and hour of irradiation; (2) by looking for vaginal plugs, which, in the majority 

 of cases, represent fertilizations between 1 :00 and 4:00 a.m., so that, under this system, 

 his "first day" approximately equals day 134 to 1%, depending on hour of irradiation. 

 It is thus obvious that within a given one of his stage groups, pregnancies may differ 

 in age by a whole day. In general, the designation of his "first day" as day one is 

 probably representative of the majority of his cases and will be followed in this review. 



