English: A contraction wave is depicted propagating from one cell to the next in an epithelial layer of cells. This initiates differentiation of the cell, which starts to produce cell type specific molecules, shown in red. If these molecules increase in number over time, as the wave propagates, a gradient of the molecules will develop across the epithelium, especially because differentiation waves propagate slowly, taking hours to cross it. The differentiation has already occurred, so the gradient is not causal of differentiation. In fact, the gradient may be called an irrelevant epiphenomenon.