Eggs of the sea urchin Strongylocentrotus purpuratus were examined by indirect immunofluorescence microscopy for tubulin-containing structures at intervals from fertilization through first cleavage. The staining revealed that the monaster is made up not only of the sperm aster but also of tubulin-staining fibers originating elsewhere in the egg. The monaster does not divide directly but is broken down first before the amphiaster or interphase asters begin to form. The interphase asters reach a peak of development at the streak stage and are in turn broken down before the formation of the mitotic apparatus. The breakdown of the monaster, interphase asters, as well as the asters of the mitotic apparatus proceeds from the cell center or aster centers to the periphery of the cell and is followed by growth of new asters, also proceeding outward from the aster centers. The pattern suggests a transient wavelike movement of some condition, or factor, which favors microtubule depolymerization.

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