During embryogenesis, for a number of cell cleavages (the specific number depends on the type of organism) all the cells of an embryo will be morphologically and developmentally equivalent. This means, each cell has the same development potential and all cells are essentially interchangeable, thus establishing an equivalence group. The developmental equivalence of these cells is usually established via transplantation and cell ablation experiments. As embryos mature, more complex fate determination occurs as structures appear, and cells differentiate, beginning to perform specific functions. Under normal conditions, once cells have a specified fate and have undergone cellular differentiation, they generally cannot return to less specified states; however, new research indicates that de-differentiation is possible under certain conditions including wound healing and cancer.