Plus-end-directed motor proteins cross-link the overlapping, antiparallel, overlap microtubules and slide the microtubules past each other, thereby pushing the spindle poles apart. The red arrows indicate the direction of microtubule sliding. Minus-end-directed motor proteins bind to the cell cortex and to those astral microtubules that point away from the spindle and pull the poles apart. These astral microtubules shorten as the spindle poles are pulled toward the cortex.