Studies of Unequal Cleavage in Molluscs II. Asymmetric Nature of the Two Asters

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Abstract
By observing living eggs of Spisula solidissima with video polarized light microscopy, we confirmed that the metaphase spindle with its two asters (one slightly smaller than the other) forms at the center of the cell and then migrates to one side. The direction of spindle migration is presaged by the tilted apposition plane between the two pronuclei at an earlier stage. Spindle migration is apparently brought about by a centering movement of its larger aster; the smaller aster behaving as though it were pushed against the cortex, its rays being bent flat. After migration, the spindle undergoes a rocking motion pivoted on the centrosome of the central aster, as through “searching” for an anchorage site for the peripheral centrosome. Eventually the peripheral centrosome becomes fixed to the cortex on the side of the future AB-blastomere, about 30°higher than the equator. Thereafter, the cell cleaves asymmetrically producing the smaller AB- and larger CD-cell. The unequal cleavage is thus achieved in two steps: (l) centering of the larger aster and (2) “searching” oscillation and eventual contact of the centrosome of the smaller aster to a specific site on the cortex.