AbstractDividing eggs ofEchinarachnius parma, Hydractinia echinata, Cerebratulus fuscusandMytilus eduliswere subjected to experiments designed to yield additional information concerning the cytoplasm in the cleavage plane. Answers to the following questions were sought: (1) Will the base of the furrow cut itself upon a needle placed in its path at right angles to the cleavage plane? (2) What part of the surface is responsible for the constricted shape of the partly divided egg? (3) Does completion of furrowing depend upon a specific subsurface cytoplasmic arrangement? It was found that in no case could the base of the furrow cut itself upon a needle. The constricted shape of the partly dividedEchinarachniusegg depends on the integrity of the surface at the base of the furrow. The eggs of all four species completed division despite drastic stirring and mixture of subsurface cytoplasm which was begun during early stages of furrowing and continued until the blastomeres were separated.