Developmental biology is a theory of interpretation. Developmental signals are interpreted differently depending on the previous history of the responding cell. Thus, there is a context for the reception of a signal. While this conclusion is obvious during metamorphosis, when a single hormone instructs some cells to proliferate, some cells to differentiate, and other cells to die, it is commonplace during normal development. Paracrine factors such as BMP4 can induce apoptosis, proliferation, or differentiation depending upon the history of the responding (...) cells. In addition, organisms have evolved to alter their development in response to differences in temperature, diet, the presence of predators, or the presence of competitors. This allows them to develop the phenotype, within the limits imposed by the genotype, best suited for the immediate habitat of the organism. Most developing organisms have also evolved to expect developmental signals from symbionts, and these organisms develop abnormally if the symbiont signals are not present. Thus Hoffmeyer’s “vertical semiotic system” of genetic communication and “horizontal semiotic system” of ecological communication are integrated during development. (shrink)