The purpose of this investigation was to determine the capacity for furrow establishment of the mitotic spindle in several kinds of dividing animal cells. To this end the cell shape was altered so that equatorial surface was brought in contact with the spindle and its normal geometrical relation with the asters changed. In flattened Tripneustes gratilla eggs, furrows formed a t the tips of deeply cut notches that impinged upon the spindle area. In Echinarachnius parma eggs, furrows formed at the tips of deeply cut notches and also at the margins of perforations that straddled the spindle. The distance separating the perforations was less than the normal spindle diameter. In flattened, dividing cultured newt (Diemictylus viridescens) kidney cells furrows formed in equatorial surface that was pushed inward far enough to displace the metaphase chromosomes. These results appear to indicate that the mitotic spindle of animal cells can establish furrows. We speculate that the actual role played by the spindle in normal cytokinesis depends upon the relative sizes of the asters and the spindle and upon the normal distance between the spindle and the equatorial surface.
In spherical cells with a central mitotic apparatus, the centers of the asters are closer to the poles than to the equator. This circumstance is basic to several hypothetical explanations of the way in which the mitotic apparatus establishes the division mechanism. This investigation was designed to determine whether that geometrical relationship is necessary for division. Fertilized, mechanically denuded sand dollar eggs were inserted into glass loops, which reduced the diameter in the constriction plane from the normal 142 to 78-80 microns and partly constricted the cell into equal parts. The mitotic apparatus straddled the constriction, and its length was not significantly changed. The manipulation increased the distance from the astral centers to the poles and decreased the distance from the astral centers to the equator to a degree that reversed the normal distance relations. These cells divided normally. Ethyl urethane (0.06 M) reduces the size of the mitotic apparatus and blocks cleavage in spherical cells. When treated cells are confined in 80-microns i.d. capillaries, they divide. Treated cells also divide when they are constricted by an 80-microns i.d. glass loop if the mitotic apparatus straddles the constriction. An equal degree of constriction in the subfurrow and subpolar areas did not reverse the effect of urethane. The results demonstrate that cleavage does not depend on the normal distance relation between the mitotic apparatus and the poles, and that the urethane effect can be remedied only by reducing the distance between the mitotic apparatus and the equatorial surface. Both findings are inconsistent with the polar stimulation hypothesis.
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