The various forms of protrusions of the surface of tissue cells are described in relation to their role in cell locomotion. It is proposed that these cells have enough cell surface (plasma membrane and linked microfilamentous cortical cytoplasm) at any given moment during interphase of the cell cycle to satisfy their need for the local increases in cell surface area that accompany protrusive activity. Thus, a local increase in protrusive activity in one region of the cell surface would be accompanied by a corresponding decrease elsewhere; that is, formation of new protrusions would require retraction of other protrusions already present and the area retracted would be equivalent to the area protruded. Evidence marshaled in support of this hypothesis includes disappearance of microvilli and other microprotrusions during cell spreading, increase in protrusive activity of uncontacted regions of the cell surface during contact inhibition of cell movement, antagonism between blebbing and spreading, accelerated protrusive activity at the leading edge upon abrupt retraction of the trailing edge, surface flow during bleb formation, and antagonism between various protrusive activities associated with cell movement and cytokinesis. Finally, the relevance of these findings to two important developmental problems is explored: the commencement of gastrulation and directional cell movements during morphogenesis.