The purple urchin zygote has a diameter of approximately 80 µm. In this zygote furrowing normally initiates just after microtubule tips reach the cell equator. For a detailed description of the relationship between microtubules and myosin regulation during cytokinesis see Foe and von Dassow, 2008. Also note here and in the companion gallery showing microtubules in green urchin zygotes how, prior to anaphase onset and beginning again in telophase, microtubules in each half-cell emanate from a point - the centrosome. By contrast, during anaphase astral microtubules project back to the edge of an enlarging circle - the periphery of the centrosphere.
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