A method was developed to investigate the mechanical structure of the cytoplasm based on the movement of an intracellular gold particle subjected to centrifugal acceleration (the gold particle method). The movement of the particle in the cell was observed and recorded with a new centrifuge microscope of stroboscopic type (13). In eggs and oocytes of the echinoderms, Clypeaster japonicus, Asterias amurensis, and Asterina pectinifera, the particle moved in the cytoplasm by an applied centrifugal acceleration in the centrifugal direction, but the course was not exactly straight and the velocity fluctuated during the movement, suggesting the existence of a network structure in the cytoplasm. In fertilized eggs, the movement of the particle by the centrifugal acceleration was impeded by the structures of the sperm aster and the cleavage diaster. The apparent viscosity of the cytoplasm in fertilized eggs changed in parallel to the development of the sperm aster and the mitotic diaster in the cell. These results indicate that the asters are really rigid structures in the cell as previously shown by the magnetic particle method (8).