Mechanisms that divide cells by the action of a physically active surface require a causal link between the surface and the mitotic apparatus to account for the position and timing of furrow activity. The system requires a contraction promoter that originates from the mitotic apparatus and also requires a responsive surface. The specifics of this stillhypothetical system depend upon the nature of the proposed division mechanism. The contraction promoter is a form of stimulus, and it could have more than one possible mode of action (Greenspan, 1977). In a stable dynamic process, small causes produce small effects, and large effects are produced by an accumulation of small causes. In Bütschli's (1876) hypothesis, the additive effect of the astral rays dehydrated the equatorial surface to a greater extent than the rest of the surface and created a local increase in tension at the surface. Progressive furrowing required progressive increase in tension at the surface, which, in turn, required more dehydration. As Bütschli described the process, the continuing presence of the mitotic apparatus was required for completion, because each increment of tension increase required an increment of dehydration. In contrast, an unstable dynamic process requires only a relatively small perturbation to set in motion a system that is constituted in a way that amplifies the effect of the trigger.