The data obtained by experimental studies on both cleavage and tissue cells indicate that the forces that deform cells during division originate at the surface and cortex and that they are contractile in nature. Much of the energy and ingenuity that has been expended in the studies of dividing cells was directed toward the identification of the physical mechanism. Inability to distinguish between cause and effect permitted divergent interpretations, but the observations generally have been reasonably accurate and useful. Qualitative data excite the imagination, but quantitative data are often necessary to make a point.

Duration of the Functional Period

The period from the first indentation at the equatorial surface to the complete separation of the daughter cells is about 8 minutes in normal cleaving sand dollar (E. parma) eggs. The period between cleavages is 48 minutes, so that active furrowing comprises about 17% of the cycle. On the other hand, in some large eggs with relatively small, highly eccentric mitotic apparatuses and, consequently, unilateral furrows – such as the eggs of amphibians and some ctenophores – the second cleavage begins before the first is completed. In these cases, active furrowing is in progress throughout the entire cycle. In order to understand whether the relatively brief functional life of the sand dollar furrow results from intrinsic factors, or from the milieu in which it operates, or from the consequences of its activity, it is necessary to understand the factors that begin and end division activity.