摘要：

Normal embryogenesis of the vertebrate nervous system, like the development of numerous other organ systems, is often associated with the death and resorption of large numbers of cells ( Glücksmann , 1951; Saunders , 1966). In the developing brain two main periods of cell death can be distinguished ( Kllen , 1955). The first occurs during the earliest morphogenetic stages of development (i.e., from the time of formation of the neural plate and tube through the neuromeric stage), when the proliferating neuroepithelium undergoes a series of cellular movements that mold the basic form of the definitive neural apparatus. The second period of necrosis occurs later, during the histogenetic phase of neural ontogeny, when the young nerve cells migrate from the ependymal zone and undergo their cytological differentiation. Thus, cell deaths consistently appear within the undifferentiated neuroepithelium during invaginations, evaginations, separations of rudiments or seam formations (classified as "morphogenetic" degenerations by Glücksmann , 1951), and neuronal cells also tend to degenerate during the period of axodendritic outgrowth (classified as "histogenetic" degenerations by Glücksmann , 1951). A third class of cell death results in the complete regression of a vestigial organ, such as the degeneration of the region in mouse and pig embryos that is homologous to that of the paraphysis of more primitive vertebrates ( Ernst , 1926; Froboese , 1926). This "phylogenetic" type of necrosis (after Glücksmann , 1951) will not be considered in detail here.

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