Abstract
1. 1. Mitotic spindle and asters were removed from eggs of the heart urchin, Clypeaster japonicus by sucking them into a micropipette inserted into the egg. 2. 2. Cleavage can take place when the spindle is removed during anaphase or later. In some eggs, the initiation of furrowing is seen when both spindle and asters have been removed during metaphase. 3. 3. The position of the cleavage plane is unaltered in the above case as well as when the mitotic figure is displaced by the removal of a part of egg protoplasm. 4. 4. It is concluded that furrowing results from the active function of the cell cortex which, in turn, is under the influence of the internal structures (nucleus, spindle or asters).
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