Aster‐equatorial surface relations and furrow establishment.

Flattened eggs of the sea urchin Hemicentrotus pulcherrimus were used in experiments designed to determine whether furrow establishment depends upon any particular geometrical relationship between the asters and the equatorial surface. In normal eggs at first cleavage the asters are 29.5 μ apart and the distance from the spindle center to the equatorial surface is 47.3 μ. In flattened eggs, the mitotic apparatus was pushed to an excentric position. The maximum spindle‐to‐surface distance that permitted furrowing was 62.5 μ. At second cleavage the spindle‐to‐surface distance is normally 36.2 μ but furrows develop when the distance is as great as 47.5 μ. The interastral distance and spindle‐to‐surface distance were varied simultaneously in flattened, dispermic eggs containing four asters. The activity of supernumary sperm asters was not detectably different from that of the mitotic apparatus. If the interastral distance was 32.5 μ furrows formed when the spindle‐to‐surface distance was between 32.5 μ and 47.5 μ. When the interastral distance is increased to 35 μ or more at the same spindle‐to‐surface distances, furrowing was rare. Very short spindle‐to‐surface distances were produced by perforating the flattened cells. They allowed study of combinations involving large interastral distances and short spindle‐to‐surface distances. Under this circumstance asters 35 μ or farther apart invariably elicited furrows when the spindle‐to‐surface distance was 20 μ or less. Decreasing the spindle‐to‐surface distance remedies a deficiency occasioned by abnormally great interastral distances. These results are considered to support the view that furrow establishment requires conjoint action of a pair of asters upon equatorial cell surface.