Mitotic tissue-culture cells of the newt Triturus viridescens were irradiated in various cytoplasmic regions with monochromatic or heterochromatic ultraviolet microbeams (conical, with 25° half-angle, 8 μ in diameter in focal plane). Depending on exposure, some spindles were destroyed (i.e., true anaphase was prevented), and this was sometimes followed by quasirosette formation and false anaphase. Efficiency of spindle destruction was not detectably affected by the presence of a centrosome or part of the formed metaphase spindle in the irradiated region. For 280-nm irradiation the exposure which destroyed 50% of spindles was$2\times 10^{12}$ in terms of incident photons per cell times the cell depth in microns. Direct irradiation of centrosomes did not impair their mutual attraction for kinetochores. In metaphase-irradiated cells which survived spindle destruction, delay of true anaphase was critically dependent on whether irradiation occurred before or after the arrival of the last kinetochore at the spindle equator, the delay being strikingly longer in the latter situation.

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