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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.