Perhaps the most dramatic, seemingly miraculous aspect of human life is the transformation of a one-celled fertilized egg into an independent, fully developed individual. From the beginning of recorded thought, people have pondered how a baby forms in the mother's body and how two parents can produce another human being who, although unique, possesses characteristics of each of them. Aristotle dissected the embryos of various birds, established their sequence of organ development, and speculated that the hereditary traits of a child resulted from the mixing of the male's semen with the female's menstrual blood. The human egg was first observed and the modern science of developmental biology was born in the nineteenth century. In its broad sense, developmental biology embraces the scientific study of changes in form and function from fertilized egg through old age–which is the scope of this chapter.

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