The present paper deals with the alteration of cell-population of the MTK-sarcoma III caused by X-irradiation. The X-rays were given to rats with whole body exposure at a dose level of 1,500r or 500 r on the 3rd day of transfers of the tumor. Samplings of tumor cells for cytological study were made from the treated animals at 72 hours after X-irradiation, and the tumor cells were colored either with acetic dahlia or acetic orcein by means of the water pretreatment squash method. The occurrence

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... f two tumor-cell populations was remarkable in the MTK-sarcoma III One consists of tumor cells having the diploid number at 96.2 per cent and the other is those having near-tetraploid number at 3.8 per cent. When the tumor was irradiated with X-rays at a dose level of 1, 500 r, tumor cells showing a near-tetraploid chromosome-number mode appeared at about 40 per cent. In irradiated rats, the tumor cells containing 40 chromosomes showed 34.6 per cent, while those having 76 chromosomes was 9.25 per cent in frequency. It is noticeable that about 20 per cent of tumor cells after irradiation were characterized by outstandingly large marker M-chromosomes which have not been found in tumor cells of untreated rats. It is further interesting to note that the near-tetraploid cells showed an increase in population after X-irradiation. The fate of X-ray-induced tetraploid cells was studied: A part of tumor ascites after irradiation was transplanted into untreated rats, and the other part was inoculated into the rats irradiated previously with X-rays at 1,500r or 500r. The results showed that the neartetraploid cells gradually decreased in number during a period from 1 to 7 transfer generations, while the tetraploid cells survived rather longer in the irradiated rats than in the untreated ones. It seems likely that the tetraploid cells induced by X-irradiation may be able to favourably grow under a new physiological condition of host animals induced by X-rays . Fuji Women's College, Sapporo Res. 11 : 613-629. Makino, S. 1957 The chromosome cytology of the ascites tumor rats, with special reference to the concept of the stem cell. Intern. Rev. Cytol. 6 : 25-84.