A model is proposed in which stimulation of cortical cytoplasm occurs near the distal ends of astral rays. Levels of stimulation sufficient to cause furrowing occur only in equatorial zones between asters. The model can account for positioning of furrows in very large cells (fertilized eggs of amphibians, birds, and fish) and in cells with several mitotic apparatuses (insects). Finally, the model correctly predicts the positioning and occurrence of furrowing in two experiments in which cellular shape was manipulated into either an hourglass or a cylindrical form before division. These results are consistent with equatorial stimulation theories in which mitotic asters differentially stimulate the future furrow region (equatorial cortex). The results are not consistent with models requiring differential stimulation of nonfurrowing, polar regions of the cell.

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