Figure 4.01 - Schematic illustration of the sensitive or critical periods in human development. During the first 2 weeks of development, the embryo is usually not susceptible to teratogens. During these pre-differentiation stages, a substance either damages all or most of the cells of the embryo, resulting in its death, or it damages only a few cells, allowing the embryo to recover without developing defects. Filled bars denote highly sensitive periods; open bars denote stages that are less sensitive to teratogens.
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