1. Exposure of the testes of rats to 100 r of X-rays at birth inhibits the division of gonocytes but does not prevent them from differentiating into transitional cells. The latter, however, fail to divide to form spermatogonia type-A, but increase markedly in size so that by 8 days post partum (p.p.) the testes of irradiated animals contain numerous giant transitional cells which are destined to degenerate.

  2. The metabolic activity of germ cell nuclei in normal and irradiated rats has been studied by the technique of autoradiography, using radioactive precursors of DNA, RNA and protein.

  3. The results have demonstrated that synthetic processes continue in cells whose division has been blocked. The synthesis of RNA is largely unaffected by irradiation. DNA synthesis may be prolonged after exposure, but the amount of 3H-thymidine incorporated by the nucleus declines from 5 days p.p. and does not keep pace with nuclear growth. The synthesis of nuclear protein, on the other hand, increases from 5 days after irradiation and the quantity of phenylalanine taken up by giant cells seems to be proportional to nuclear size. Thus the accumulation of nuclear protein appears to be the main factor in nuclear enlargement.

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