Clone 515 from an established tumour cell strain (human osteochondrosarcoma) wasplanted in Carrel flasks with small cover slips. After 24 hours' incubation, the flasks withplanted cells were treated with Co~(60) or P~(32). Untreated cells were cultivated as controls.Daily preparations of the treated and untreated materials of 7--9 successive days ofincubation were made and stained by Feulgen after fixation in Carnoy's solution. Mito-tic index, normal and abnormal mitotic figure counts were then made. The mean values obtained from 5 lots of the untreated material showed that thedaily mitotic indices varied from 1.5% to 2.5%. Spontaneous abnormal mitosis con-sisted of lagging chromosomes in metaphase and anaphase, anaphasic bridge, unequaldivision, multipolar and multipolar with lagging chromosomes, chromosome fragmentationand chromosome stickiness. All these appeared in all fixed daily specimens. The abnor-mal mitotic rate increased with the time of cultivation. The most frequent abnormalitywas lagging chromosomes found during metaphase. Cells after being treated with 1000 r Co~(60) or 5?c P~(32) showed a temporary suppres-sion of mitotic index, then the mitotic rate increased and then dropped. The effect ofP~(32) was not so striking as that of Co~(60). After Co~(60) irradiation the abnormal figures weremainly chromosome fragmentation, while after P~(32) the abnormality was chiefly chromo-some stickiness. Thus, tumor under successive cultivation with or without irradiation showed dif-ferent types of mitotic abnormalities. The causes of the appearance of the abnormalitieswere discussed.

A comparision of the ECa 109 esophageal epithelial carcinoma cells and normal esophageal epithelial cells was made. Electron microscopically these two cells revealed remarkable differences. The normal esophageal epithelial cells had narrow intercellular space, fewer microvilli, more desmosomes and tonofilaments, with small and round nucleus. No polyribosome could be seen. The ECa 109 cells had wider intercellular space, more microvilli, fewer desmosomes and tonofilaments, larger and irregular nucleus, large and distinct nucleolus. Besides, the appearance of polyribosome was characteristic of cancer cells. Thus, the following characteristics were used as makers of cell differentiation: 1. less microvilli; 2. abundant tonofilaments; 3. more desmosomes; 4. monoribosomes; 5. round nucleus; 6. small nucleus/cytoplasma ratio.Upon addition of 2％ DMSO ECa 109 under went striking morphological changes resembling the terminal differentiation stage of normal esophageal epithelial cells.Inaddition, after DMSO treatment, circular Golgi complexes surrounded by small vesicles could be seen. Small vesicles and primary and secondary lysosomesappeared too. Electron-dense particles with halo were easily seen in the nuclei.