Chromosome rearrangements induced in the mouse by embryonic X-irradiation. II. Irradiation of male foetuses at the 13.5th day of gestation

Pregnant females were exposed to 0, 100, 200 or 300 R of whole body X-irradiation at day 13.5 of pregnancy. Except for the dose of 300 R where only 7 males were used, 10 males chosen from the litters of different females were sacrificed 100 days after birth and for each testis 100 dividing spermatocytes were examined for the presence of chromosome rearrangements. No chromosome rearrangement could be detected in the spermatocytes of adult male mice irradiated at the foetal stage. On the other hand dividing spermatocytes are known to carry translocation configurations after exposure at the spermatogonial stage in the adult male mice. Thus large differences exist in this respect between the dividing gonocytes of the foetal male mouse and the type-A spermatogonia of the mature one.