In order to analyze the cellular mechanisms of shape formation, the shape of individual 3T3 cells was perturbed by micromanipulation resulting in the detachment and relaxation of a cellular extension and the bending of the extension to form an "elbow" at a variable angle beta. Finally, the tip of the extension was allowed to reattach to the substrate away from the cell. The cells reacted by drawing the extension tight. If beta less than 90 degrees, the elbow moved laterally for 8-15 min until the extension projected orthogonally at the cell surface. If beta greater than or equal to 90 degrees, the extension remained stationary. Finally, in all cases webs formed between attachment points in the perturbed area. If the tip of the extension was allowed to touch its own cell body, thus forming a loop, the cells invariably closed the loop. The paper interprets the cellular reaction as the result of cortical tension and suggests that it is a major factor in the formation of fibroblast shape and the expressions of fibroblast motility.