RADIATION IN PRENATAL DEVELOPMENT 899 



In a class by itself is the claim by Levine (1927) that the irradiation of 

 pregnant mice lengthened the mating-to-birth interval by from 3 to 41 

 days in twenty-two of twenty-six cases. Examination of his method 

 reveals that the author did not realize that (1) conception may occur a 

 considerable interval after a male and female mouse are caged together, 

 and (2) the first litter conceived may be completely lost as a result of 

 early irradiation, so that the litter observed at birth stems from a later 

 conception. Even where errors are not as crass as this, it should be borne 

 in mind that any claim of delay must be supported by adequate control 

 data, since there is considerable variation in length of gestation period 

 depending on genetic constitution, and even individual variation within 

 an inbred strain. Moreover, suckling a previous litter may markedly 

 lengthen the gestation period of the litter being carried. This fact was 

 not known to earlier authors. 



Russell (1950) found that 88 per cent of her control mice and 80 

 per cent of the irradiated females of the same inbred strain delivered by 

 approximately 19^ days after conception. The rest of the control 

 group was born within the following 16 hours and there were no indica- 

 tions (from the developmental state of the fetuses) that the balance of the 

 irradiated animals, which was delivered by caesarean section at 19^ days, 

 would have been born later than that. There is thus no evidence of 

 delayed parturition following irradiation during the preimplantation 

 stages and the period of major organogenesis, but critical data are needed 

 to answer this question for irradiation during the period of the fetus. 



4. ISOTOPE STUDIES 



It remains to mention two recent studies in which radioactive isotopes 

 were injected into pregnant females and the effects on the resulting young 

 studied. It should be kept in mind that isotope treatment differs from 

 external irradiation in two ways: (1) the period of effective irradiation 

 may extend for a considerable time following injection; (2) the radiation 

 source may be concentrated in certain tissues or organs because of 

 chemical affinities of the isotope or other reasons. Response, therefore, 

 does not necessarily represent relative degrees of radiosensitivity through- 

 out the body. 



Burstone (1951) studied the effect of localized electron bombardment 

 produced by the metabolism of P 32 in fetal tooth primordia of inbred 

 mice. Mothers were injected 2-6 days before expected parturition with 

 5-17 juc/g and the teeth of the young studied 6-28 days after birth. The 

 following are some of the findings: (1) fetal teeth are more radiosensitive 

 than the teeth of newborns (or P 32 concentrates in them to a greater 

 degree) ; also, injections 4-5 days before term were more effective than 

 injections 1-2 days before term; (2) the development of the third molar is 

 completely inhibited, while that of the first and second molars, which had 



