Because information concerning the time relations of the cleavage stimulus should prove useful in characterizing both the stimulus and the nature of the response in the cell surface, methods were devised for determining how long the surface must be acted upon by the asters to produce a furrow and the duration of the latent period between the end of this stimulus period and appearance of the furrow. Eggs of the sand dollar Echinorachnius parma were used in experiments wherein unstimulated surface was brought close to the interastral zone of the mitotic apparatus for specified periods. Since this modification subjected the cell to physical stress or constraint, several different methods were used. As was anticipated, the times determined differed somewhat according to the method used. Exposure of the surface to the interastral zone for about one minute subsequently produces an active furrow. The latent period between the end of the stimulus period and the appearance of the furrow is about two and one-half minutes and the total time from the beginning of stimulation to appearance of the furrow is between two and three and one-half minutes.
The mitotic apparatus can repeatedly elicit new furrows within the same cell despite visible loss of distinctness from frequent handling. This suggests that stimulation is accomplished by a fundamentally simple relationship between asters and surface. During the latent period the presumptive furrow area has greater ability to retain its contour despite increased tension at the surface than does non-furrow surface. The undiminished capacity for division that persists in cells subjected to frequent and extensive cytoplasmic shifting which accompanies relocation of the mitotic apparatus by pressure displacement implies that after stimulation, completion of the cleavage process is independent of any specific subsurface cytoplasmic configuration.