Mechanical properties of the cortex before and during cleavage.  The changes in stiffness of the cell before and during cleavage reported previously for various kinds of echinoderm eggs are classified into three types. There is no general rule that cleavage starts when the stiffness attains a maximum or a minimum. Because the stiffness of the cell surface is much greater than that of the mitotic apparatus, the change in stiffness of the cell represents mainly the change in stiffness of the cell surface. The tension at the cell surface changes in parallel over the entire surface before the onset of cleavage. After the onset of cleavage, the tension at the furrow surface becomes greater in the direction parallel to the furrow than perpendicular to it. A remarkable change in mechanical properties occurs at the furrow surface simultaneously with the onset of cleavage. Judging from the temporal and spatial coincidence between this change and the formation of the contractile ring reported previously in sea urchin eggs at the onset of cleavage, it is concluded that this change is due to the formation of the contractile ring that generates the motive force for cleavage.