According to this scheme, the essential stages are (1) proliferation of a vast number of undifferentiated brain cells; (2) migration of the cells toward a predetermined location in the brain and the beginning of their differentiation into the specific type of cell appropriate to that location; (3) aggregation of similar types of cells into distinct regions; (4) formation of innumerable connections among neurons, both within and across regions; and (5) competition among these connections, which results in the selective elimination of many and the stabilization of the 100 trillion or so that remain. These events do not occur in rigid sequence but overlap in time, from about 5 weeks after conception onward. After about 18 months of age, no more neurons are added, and the aggregation of cell types into distinct regions is roughly complete. But the pruning of excess connections—clearly a process of great importance for the shape of the mature brain—continues for years.

This model of the sequence of brain development has led toward many fruitful lines of investigation in neuroscience. Among other things, it can explain well-known birth defects of the brain or the nervous system in terms of the stage at which development was disturbed. If, for instance, at a very early stage the neural tube fails to close properly, the cells that should form the forebrain and its overlying skull and scalp may not be generated; this condition, anencephaly (“without brain”), almost always results in stillbirth or in survival for only a few hours. Less severe defects of the neural tube may give rise to varying degrees of spina bifida (“split spine”), with the spinal column missing the bony protection of some of its vertebrae. Exposure to x-rays, high levels of alcohol, and some drugs can impair development at crucial early stages, as can the mother's infection with certain diseases such as rubella (German measles).

At about the midpoint of pregnancy, from about 15 to 20 weeks after conception, the number of brain cells in the cerebral cortex increases rapidly; by the seventh month, the fetus is emitting its own brain waves, which can be detected through the mother's abdomen. Several lines of evidence suggest that proper nutrition is of greatest importance for the development of the brain at this stage, although it continues to be crucial until birth and for some time afterward. Yet even when the

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