ABSTRACT The importance of cell–cell interactions in embryonic development was first described by Driesch (1891), who showed that any of the blastomeres of the 2-cell or 4-cell sea-urchin embryo is capable of forming a complete embryo if cultured in isolation; this implied that in normal development each blastomere is aware of the other and will only form a half- or quarter-embryo, as appropriate. And it was only ten years later that Spemann (1901) discovered the phenomenon of embryonic induction, recently reviewed by Gurdon (1987) and defined as an interaction in which the differentiation of one group of cells is affected by a signal from an adjacent group. Thus the significance of cell signalling during development has been appreciated for almost a century, but, as has frequently been remarked, progress in the molecular analysis of the phenomenon has been slow compared with that in the younger disciplines of, for example, immunology and molecular biology.