We think of development as a process of inexorable progressive change by which a single cell is transformed into a complex multicellular organism, made up of many tissues and organs, each with a distinctive structure and function. In any given species, the progression from the simple to the complex occurs in an orderly fashion that is repeated with little or no variation in millions of individuals, generation after generation. Barring severe genetic mutations or environmental trauma, all individuals of a species develop in identical fashion and the final products, the adult animals, resemble each other to a remarkable degree.
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