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To characterize the control mechanisms for mitosis, we studied the relationship between the spatial organization of microtubules in the mitotic spindle and the timing of mitotic events. Spindles of altered geometry were produced in sea urchin eggs by two methods: (a) early prometaphase spindles were cut into half spindles by micromanipulation or (b) mercaptoethanol was used to indirectly induce the formation of spindles with only one pole. Cells with monopolar spindles produced by either method required an average of 3 X longer than control cells to traverse mitosis. By the time the control cells started their next mitosis, the experimental cells were usually just finishing the original mitosis. In all cases, only the time from nuclear envelope breakdown to the start of telophase was prolonged. Once the cells entered telophase, events leading to the next mitosis proceeded with normal timing. Once prolonged, the cell cycle never resynchronized with the controls. Several types of control experiments showed that were not an artifact of the experimental techniques. These results show that the spatial arrangement of spindle components plays an important role in the mechanisms that control the timing of mitotic events and the timing of the cell cycle as a whole.