The purpose of this investigation was to study the nature of the immediate response of the surface to the mitotic apparatus in sand dollar (Echinarachnius parma) eggs. The normal geometrical relationship was changed by snaring fertilized eggs between opposed, overlapping hooks and constricting the eggs to reshape them into two partial spheres joined by a constricted neck about 27 Fm in diameter. When both asters of the mitotic apparatus lay on the same side of the constriction, cytoplasm flowed out of the nucleated part across the constriction, beginning about 30 min before anticipated cleavage time. After the asters moved across the constriction, the direction reversed, and reversal usually occurred several times before cleavage (n = 18). The average time between beginning of flow in one direction and beginning of flow in the opposite direction was 7.7 min (n = 23). Cells were bisected and constricted to determine the role of the mitotic apparatus. Bisected cells with two asters behaved like constricted normal cells (n = 15). Cells with no mitotic apparatus developed no cytoplasmic flow (n = 11). In cells with one aster, flow most often occurred within ten min of anticipated cleavage time (n = 14). When whole constricted eggs were treated with Cytochalasin B at the minimum concentration that blocks cleavage in normal eggs (3 pg/ml), neither cytoplasmic flow nor cleavage occurred, although the mitotic apparatus developed normally.Cytoplasmic flow in constricted eggs in cleavage stages requires asters and functional actinassociated cytoskeleton. The results suggest that the asters cause an increase in tension in nearby surfaces.The idea that the role of the mitotic apparatus in cytokinesis of animal cells is confined to preliminary modification of regional surface properties was postulated in the last century by Biitschli (1876) and then by Ziegler in 1903. Later, demonstrations of the cell's ability to complete division despite removal of the mitotic apparatus (Yatsu, '12; Hiramoto, '56) led to general acceptance of the idea. The nature of the immediate effect of mitotic apparatus action on the surface can not be determined by simple observation, and two alternative mechanisms have been suggested. Lillie ('031, Wolpert ('601, Schroeder ('81), and White and Borisy ('83) proposed that the mitotic apparatus causes a decrease in tension or relaxation of the affected surface, while Biitschli (18761, Ziegler ('03), and many others have proposed that the mitotic apparatus causes an increase in tension or contraction in tfhe affected surface. Either response could lead to the organization of the filamentous contractile ring (Arnold, '68; Schroeder, '681, which, it is generally agreed, bisects the cell by constriction.Measurements of the changes in cytoplasmic stiffness that appear to accompany the cell cycle (Hiramoto, '63; Yoneda and Dan, '72) have not provided a basis for eliminating either of the hypothetical mechanisms. The understanding of the relation between the stiffness changes and cytokinesis is complic...