 What controls the path taken by a cell or piece of developing tissue—a pre-determined genetic program or ongoing orchestration of stimuli? Waddington (1939, 1940, 1957) identified genes as the determinants of epigenetic landscape topography. This is depicted in a companion diagram showing the landscape’s “underside” as a network of interacting biochemical products “which are ultimately controlled by genes” (1957: 36). Waddington first articulated the landscape analogy in 1939, as a generalization of time- and dose-effect curves representing the role of genes in producing specific phenotypic effects. The landscape model results from including interactive effects of the entire genome on a particular pathway: