Abstract: Proposed mechanisms for the directed, morphogenetic movement of cells and establishment of defined cell-cell contacts during embryogenesis include molecular gradients and selective cell adhesion (8, 31,37). Pioneered by the classic studies of H. V. Wilson on reaggregation of dissociated marine sponge cells (42–44), investigations of cell sorting in vitro have demonstrated cellular recognition and/or selective adhesion at the tissue, organ, and species levels in a variety of developing metazoan systems (14, 15, 20, 25, 34). Although there is increasing evidence that developmental regulation of cellular affinities is mediated by components of the cell surface (1, 2, 16, 18, 30, 35), the biochemistry of intercellular recognition sites remains largely obscure.