Rheological properties of echinoderm eggs during cell division.

Mechanical properties of the cell before and during cleavage in sea urchin eggs (a typical equal symmetrical cytoplasmic division) and polar-body formation (a typical unequal cytoplasmic division) are reviewed with special reference to the mechanism of cell division. Both in sea urchin eggs and starfish oocytes, the tension at the cell surface gradually increases before the onset of cytoplasmic division. The tension gradually decreases during division in starfish oocytes and in some sea urchin eggs. Some other sea urchin eggs display two peaks of the tension, one just before the onset of division and the other during division. The cleavage furrow is formed by the active contraction of a layer formed in the equatorial cortex of the cell in sea urchin eggs and of a ring-shaped layer formed in the cortex around the animal pole of the cell in starfish oocyte.