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Cortices of sea-urchin eggs were studied by electron microscopy to identify the structure responsible for the rise in tension at the egg surface prior to cleavage. During anaphase the tension increased and fine filaments of 70-90 A in diameter appeared in the cell cortex forming a thin meshwork beneath the cell membrane. The meshwork spread all around the egg cortex without reference to the mitotic axis and the number of filaments seemed to increase up to telophase. Immediately before appearance of the cleavage furrow, the meshwork in the anticipated furrow region became dense. As the furrow appeared the tension began to decrease and the meshwork disappeared. In the progressing furrow region fine filaments of the same size as that of the meshwork-filament were oriented in a bundle to form a contractile ring. Treatment with cytochalasin B suppressed both the tension increase and the formation of the filamentous meshwork. These results suggest that the component filament of the meshwork is an actin microfilament, and that the tension increase at anaphase is due to formation of a meshwork of actin microfilaments from which a contractile ring is subsequently derived at late telophase.Most animal cells tend to round up when they are ready to divide. Cells such as sea-urchin eggs that are already spherical even in interphase increase their resistance to deformation as they prepare for cleavage. Quantitative data on such mechanical behaviour of precleavage cells have been obtained largely from egg cells, and described in terms of a rise in either the 'stiffness' or 'tension' generated in the egg cortex. Most results on amphibian and echinoderm eggs observed by either a compression method, suction method or sessile-drop method indicate that the stiffening of the egg surface starts well before the appearance of the cleavage furrow (1-7). How this change in the mechanical property is brought about has not been fully explained.On the other hand, ultrastructural observations on dividing animal cells have demonstrated a bundle of microfilaments called a contractile ring in the cell cortex beneath the cleavage furrow (8-19). This circular bundle of microfilaments is believed to exert the mechanical force of cleavage (cf. 1 l), but it is not obvious how the bundle of microfilaments is formed in a specific region of the cell in a particular stage of mitosis, either by direct polymerization of protein monomers or by alignment of microfilaments.A precleavage rise in the cell tension leads us to suggest that structural elements responsible for the force of the furrow constriction may be formed in the cortex prior to cleavage. In this study, the cortices of sea-urchin eggs preparing for cleavage were examined to identify these structures.