The programmed development of the embryo involves the specification of cells in space and in time. Specification of cells according to their location is evidently achieved by a process of cell interaction and the recognition of relative cell position. Some positional cues function by setting up within a single cell asymmetries of organization that result in intercellular differences after mitotic division. Others operate within a founder population of homogeneous precursor cells, generating variety via inductive or graded diffusional signals of various sorts (reviewed for various species by Slack, 1983; Johnson & Pratt, 1983). The molecular nature of the cues that convey positional information is known for few, if any, developmental systems, but the available evidence suggests that the cues themselves are unlikely to be highly specific informational molecules. Rather, specificity is achieved through the interaction of positional signals with target cells ‘primed’ to respond to them.