Microtubules and a contractile ring of microfilaments associated with a cleavage furrow.

A ring composed largely of microfilaments is situated underneath the pellicle and at the base of the cleavage furrow in the ciliate Nassula during binary fission. The microfilaments have diameters ranging from 4 to 10 run. There are substantial indications that the ring actively constricts in a sphincter-like fashion and is the main contractile agent causing furrowing. As cleavage proceeds the ring thickens and the dense layer of the pellicle becomes progressively more deeply folded. The longitudinal axes of the folds are at right angles to the longitudinal axes of the microfilaments and the plane of the ring. Folds form only where the pellicle overlies the ring. Two distinct phases of cleavage have been distinguished. The furrow constricts the organism at a progressively more rapid rate until the cleavage constriction has a diameter of about 5 /im and the microfilaments plug the constriction. After this furrowing proceeds much more slowly. A girdle of several thousand microtubules embedded in a densely staining material forms between the ring and the pellicular folds during the final stages of cleavage. Constriction and severance of the narrow cleavage constriction joining daughter oiganisms during the final phase of cleavage involve mechanisms different from those acting during the earlier phase of furrow development.