Abstract
The relationship between the microtubules and a massive system of vesicles associated with the sea urchin mitotic apparatus was examined by light and electron microscopy. Astral rays made up of elongated vesicles and associated tracts of microtubules continue to grow toward the cell surface during late anaphase and telophase at the same time the aster center appears to be breaking down in preparation for the next division. On the basis of current knowledge of the requirements for microtubule polymerization and the known presence of a calcium-dependent ATPase in the mitotic apparatus, it is proposed that the vesicle system functions to control the polymerization and depolymerization of microtubules by calcium ion regulation. A model for such a system is proposed.
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