FIGURE 8.2Prenatal Development and Periods of Critical GrowthDuring prenatal development, individual structures form and are fine-tuned at different times. As each structure is being established, it is particularly vulnerable to alterations. Once their critical periods are complete, the structures are fully established.

LEFT: OMIKRON/SCIENCE SOURCE; CENTER: ANATOMICAL TRAVELOGUE/SCIENCE SOURCE; RIGHT: NEIL BROMHALL/SCIENCE SOURCE