Tn sea-urchin eggs, once karyokinesis reaches metaphase or anaphase, the cleavage furrow can be formed even if the mitotic apparatus is destroyed (Swann & Mitchison, 1953) or removed (Hiramoto, 1956). A similar result was obtained in frog eggs (Kubota, 1966). In amphibian eggs a much longer time is available for performing experiments than in sea urchins as the furrow first appears at the animal pole and slowly travels toward the vegetal pole. Taking advantage of this situation, Waddington (1952) and Dan & Kuno-Kojima (1963) performed various kinds of operations to elucidate the roles of the egg cortex and the inner cytoplasm in furrow formation, and Selman & Waddington (1955) also made cytological observations of the process.

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