The observations and experiments described in Chapter 3 demonstrated that no deep subsurface structures are required for division and infer that the force that drives the mechanism is located in the surface or cortex. The findings lead immediately to two further questions: First, how does the division mechanism work? and, second, how is division activity correlated in time and position with the events of mitosis? This chapter is concerned with the first question.

The typical deformations of dividing animal cells require an unequal distribution of forces, and parsimonious hypotheses require only a single force exerted in a pattern of active and passive areas. In spherical cells, either active expansion or active contraction appear to be the logical alternatives. Cell constitution and behavior seem to admit the possibility that different cells could use different division mechanisms, but at this time only one mechanism has been identified in all the animal cells that have been subjected to experimentation. For purposes of speculation, it has been customary to assume that all cells are reshaped into spheres before the events of division begin, but this generalization is not true, especially in tissue cells. Different force-producing mechanisms may require that the site of major physical activity be located in different regions.