On the rate of movement of the cleavage stimulus in sand dollar eggs

The rate at which the cleavage stimulus moves from the mitotic apparatus to the cell surface was calculated from measurements made on flattened, fertilized Echinarachnius parma eggs with excentric nuclei. The time interval between the appearance of furrows in the closer and more distant equatorial margins is proportional to the difference in the distance between the axis of the mitotic apparatus and the two surfaces. The calculated rate of stimulus movement is 6.3 ± 1.8 μ/minute. This rate is slower than that of free diffusion, and the usually studied examples of cytoplasmic streaming; it approximates that reported for microtubule growth.