[en] Mice of 13th day in-utero were irradiated with 10 R x-ray and then the cerebral undifferentiated nerve cells were compared to those irradiated with 25 R x-ray 1 - 5 hours after the irradiation. The site observed in this study was the parietal pallium of the cerebral frontal plane between optic chiasma and infundibulum of the mice in-utero. Appearance of cytoplasmic inclusion body and pyknotic cells was examined in more than 500 cells per a piece of the cerebral pallium. In addition, the appearance of karyokinetic cells was examined in the same piece. The appearance rate showed a slight increase 3 hours after irradiation in the case of cytoplasmic inclusion body and 3 - 5 hours after in the case of pyknotic cells. A karyokinetic index declined to 0.35 or lower than that of 0.78 of control group, but it returned back to the same level with that of the control group 2 hours after. By electron microscope, cells with cytoplasmic degeneration were observed 2 hours after the irradiation as seen in the case of 25 R irradiation. Necrotic cells appeared 3 hours after the irradiation. Moreover, the cells which phagocytize necrotic cells were identified, some of which had a clear contour of phagocytized necrotic cells. Five hours after the irradiation, the contour of many necrotic cells became unclear showing some destructed images. Phagocytosis of necrotic cells tended to appear earlier than that in the 25 R irradiation group, but the time of the appearance was influenced by not only the in-utero age of irradiation but also the dose of radiation. (Iwagami, H.)