We present a model that attempts to explain some aspects of cytokinesis in animal cells. We propose two separate phases of cytokinesis. The first is not dependent on the presence of the mitotic apparatus and involves a general activation of cortical contractile elements resulting in the development of a surface tension. In the second phase the asters of the mitotic apparatus interact and modulate the activities of the tension generating elements in the cortex to produce gradients of surface tension with the highest values being at the equator. Tension generating elements are assumed to be free to move in the plane of the cortex so that they will consequently move up the gradient of tension and accumulate as an equatorial belt of oriented elements i.e. the contractile ring. The model was simulated on a computer and is capable of reproducing some of the wide variety of cleavage configurations that are observed.