Essential Genetics 6th Edition by Daniel Hartl

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Programmed cell death (apoptosis) is responsible in part for shaping many organs and tissues in normal development. Consider a group of cells in the duck leg primordium that normally undergo apoptosis. If the cells are transplanted from their normal leg site to another part of the embryo just before they would normally die, they still die on schedule. But if the same operation is performed a few hours earlier, the cells do not die. How can you explain this observation?