A sheath consisting of filaments 50-70 A in diameter has been demonstrated in association with the expanded, leading margins of the cleavage furrow in unilaterally and symmetrically cleaving eggs of a jellyfish and a polychaete worm, respectively. The observations suggest that the filament system might provide a structural basis for the existence of the contractile gel that, according to a hypothesis by Marsland and Landau, accomplishes cleavage. The filamentous sheath is present only in the furrow region and is arranged in an arcuate manner in unilaterally cleaving eggs and circumferentially in symmetrical cleavage. The filaments appear to be of finite length, and a number of them must overlap to span the length of the furrow. Contraction may be accomplished if the filaments slide relative to each other. However, contraction per se was experimentally not demonstrated in the studied systems. The disappearance of microvilli and the merocrine type secretion of mucoid droplets at the interdigitating or "spinning" membrane region of unilateral cleavage suggest that the unfolding of a pleated membrane and the insertion of intracytoplasmic membranes might contribute, at least in part, to the necessary extra cell membrane.