Reversal of chemical cleavage inhibition in echinoderm eggs

Many agents will, in low concentrations, reversibly block cytokinesis without seriously affecting mitosis. Frequently such agents visibly reduce the size of the mitotic apparatus. The purpose of this investigation was to determine whether failure to divide as a result of this treatment is causally related to reduction in size of the mitotic apparatus. Ether and ethyl urethane were used as blocking agents. Hemicentrotus pulcherrimus eggs fail to cleave when placed in either 0.75% ether or 0.06M ethyl urethane in filtered sea water 30 minutes or more before cleavage time. Both agents reduce the asters. When eggs are constricted by partial removal of the fertilization membrane, the mitotic apparatus and surface are brought closer together. Constricted eggs developed furrows in concentrations of blocking agents that prevented cleavage in spherical eggs. Furrows were permanent when they coincided with the plane of artificial constriction. Cleavage in Echinarachnius parma eggs is blocked by 0.052 M ethyl urethane. The behavior of constricted, treated eggs is similar to that of H. pulcherrimus eggs. When the reduced mitotic apparatus of the treated cell is pushed to an excentric position, well‐developed but usually temporary furrows appear. Hexanediol augments the in vivo mitotic apparatus. When eggs are exposed to combined treatment of 0.25% hexanediol and 0.052 M ethyl urethane about half will, in favorable cases, develop furrows. About half the furrows are permanent. In otherwise normal eggs hexanediol treatment slightly increases the maximum spindle‐to‐surface distance that permits furrowing. It appears that the cleavage blocking effects of ethyl urethane and ether in these experiments were partially reversed by measures that restore a more nearly normal geometrical relationship between the reduced mitotic apparatus and the surface.