Schematic illustration of the critical periods in human development. During the first two weeks of development, the embryo is usually not susceptible to teratogens. During these predifferentiation stages, either a substance 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. SOURCE: Moore, K.L., The Developing Human: Clinically Oriented Embryology, Third Edition, Philadelphia, W. B. Saunders Company, 1982. Reprinted with permission.