This study examines the mechanism of cell interactions during embryogenesis. We propose to study chick neural retina cells at three levels: to examine the kinetics of cell interations at the cellular level in order to study processess that initiate and stabilize an adhesion; to isolate, solubilize, and fractionate membranes along with enzyme and labeling studies to classify the prevalent molecules in the membrane; and with immunological studies to couple selective absorption of Fab fragments to adhesion inhibition, we propose to relate membrane molecules to an adhesive function. This proposal also includes studies on the histocompatibility antigens of the mouse. We propose to examine the ontogeny of appearance of antigens on different tissues. Further, we will test histocompatibility for a possible functional role in cell interactions.