The degeneration and death of cells is a normal occurrence in renewing cell systems in mature animals. Cell death is also a frequent event during embryonic development and appears to be a normal, and perhaps necessary constituent process of morphogenesis (see Glucksmann (1951) and Saunders (1966) for reviews). The events of cell degeneration and death during ontogenesis have been observed in many developing tissues, even those which have a relatively static cell population in the adult, such as the nervous system (Hamburger & Levi-Montalcini, 1949).

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