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Developmental processes (Ensminger, P. in Lerner and Lerner, 2004 p. 1211)

Developmental processes are the series of biological changes associated with information transfer, growth, and differentiation during the life cycle of organisms. Information transfer is the transmission of DNA and other biological signals from parent cells to daughter cells. Growth is the increase in size due to cell expansion and cell division. Differentiation is the change of unspecialized cells in a simple body pattern to specialized cells in more complex body pattern. While nearly all organisms, even single-celled bacteria, undergo development of some sort; the developmental process of complex multicellular organisms is emphasized here. In these organisms, development begins with the manufacture of male and female sex cells. It proceeds through fertilization and formation of an embryo. Development continues following birth, hatching, or germination of the embryo and culminates in aging and death.

History

Until the mid-1800s, many naturalists supported a theory of development called epigenesis, which held that the eggs of organisms were undifferentiated, but had a developmental potential which could be directed by certain external forces. Other naturalists supported a theory of development called preformationism, which held that the entire complex morphology of mature organism is present in miniature form in the egg, a developmental form called the homunculus. The modern view is that developmental processes have certain general features of both preformationism and epigenesis. Thus, we know that the simple cells of an egg are preformed in the sense that they contain a preformed instruction set for development which is encoded in their genes. Similarly, we know that the egg is relatively formless, but has the potential to develop into a complex organism as it grows. Thus modern developmental biology views development as the expression of a preformed genetic program which controls the epigenetic development of an undifferentiated egg into a morphologically complex adult. One modern view is that new species may evolve when evolution alters the timing of development, so that certain features of ancestral species appear earlier or later in development.

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