The Mechanism of Cleavage in Amphibian and Sturgeon Eggs.

T H E CLEAVAGE plane in animal cells and eggs is known to be determined by the position of the mitotic apparatus (cf. Wilson, 1925). A displacement of the mitotic apparatus by centrifugation (Harvey, 1935), mechanical deformation of eggs (Rappaport, 1960, 1961) or by microsurgical operation (Carlson, 1952; Waddington, 1952; Kawamura, 1960; Chambers & Chambers, 1961; Dan & Kuno Kojima, 1963) results in a corresponding change in the location of the cleavage furrow. This shows that the position of the cleavage furrow is determined by the mitotic apparatus of the cell. On the other hand, the formation and position of the furrow in the eggs of amphibians (Waddington, 1952), sea urchins (Swann & Mitchison, 1953; Hiramoto, 1956; Chambers & Chambers, 1961) and in the neuroblasts of grasshopper (Carlson, 1952; Kawamura, 1960) depends upon the presence of the mitotic apparatus only up to the stage of late metaphase or anaphase. After these stages the deepening of the furrow can occur without the mitotic apparatus. It should be noticed that the process of cytokinesis cannot be reduced to two phenomena only: the inducing action of the mitotic apparatus and a contraction and dip of the surface egg layer in the region of the furrow. As pointed out by Swann & Mitchison (1958), no contractile structure can shrink to nothing, which would happen if the complete division of the egg into two blastomeres occurred only at the expense of the contraction of the surface layer in the region of the furrow. This shows the process of division to involve, apart from the furrowing within the egg, some other phenomena related to the formation of new cell surface membranes between blastomeres (Wolpert, 1963). Thus, the division of the cytoplasm and the creation of new cellular surfaces when eggs cleave consists of three inter-related processes: (1) the inducing action