Chick embryo epithelial cells, cultured in vitro on a plane glass surface, show ruffled membrane activity like that seen in fibroblasts. When similar epithelial cells come into contact, long-lasting adhesions form, which are associated with the cessation of membrane activity. Movement of epithelial cell sheets is correlated with adhesion to the substratum of some or all of the marginal cells. Such adhesion does not occur in the absence of membrane activity. Non-marginal cells ordinarily appear to be non-adherent to the substratum; occasionally, however, they become adherent when attachments to neighbouring cells in a sheet are broken. This behaviour is accompanied by marked membrane activity. Mobilization and orientation of marginal cells are the vital steps in movements of coherent cell sheets.