EMS, a blastomere of the Caenorhabditis elegans embryo, produces body wall muscle cell-autonomously in isolation. Within the embryonic context, however, the specification of body wall muscle derived from EMS depends on inductive interactions between its daughter MS and ABa descendants that are required to overcome inhibitory interactions with other cells. The inductive events between the MS and ABa descendants are reciprocal, specifying subsequent fates in both lineages. Both induction events are blocked by mutations in the gene glp-1, known to encode a Notch-like transmembrane receptor protein.