Tension at the Surface of the Dividing Sea-Urchin Egg

Abstract

ABSTRACT Several theories have been put forward to explain the mechanics of cleavage of animal cells. According to the expanding membrane theory of Mitchison and Swann (Mitchison, 1952; Mitchison & Swann, 1955; Swann & Mitchison, 1958), the cleaving furrow is passively pushed in as a result of ‘active’ expansion of poles; while the constricting ring theory (Marsland, 1950, 1956 a, b) anticipates that the furrow region has independent power to contract. A third alternative is ‘astral relaxation theory’ of Wolpert (1960), who advocates that cleavage is initiated by relaxation of the membrane in the polar region which allows the furrow to contract.