Fig. 6. Cell fate switching in an epigenetic landscape, as portrayed by Waddington (Waddington, 1956). In this view of embryonic regulation, cell fate regulation is based on selection between pre-existing, intrinsically robust fates. The dynamics of this developmental selection is represented as a landscape with hills and valleys. The phenotypic state of a cell at any time is indicated by the position of a marble on that landscape. The marble will spontaneously roll down the valleys (stable developmental paths) leading to a distinct phenotype. The lowest points in the valleys correspond to the distinct, stable phenotypes within a given repertoire of fates that the cell may experience.