Fig. 2. Three models explaining cleavage plane induction. (A) According to the astral relaxation model the asters signal to the cell cortex near the poles, inducing it to relax. The additional two models, equatorial stimulation through the asters (B) and equatorial stimulation through the spindle midzone microtubules (C), are based on induction of the equatorial region of the cell, either by signals from the asters (B) or from the overlapping microtubules of the spindle midzone (C). White arrows indicate stimuli and are pointed towards the induced area of the cell cortex (red). Microtubules are indicated as grey lines and centrosomes as filled grey circles (see also ^|| footnote on p. 2258).