FIGURE 4.2
Prenatal Growth of the Brain Just 25 days after conception (a), the central nervous system is already evident. The brain looks distinctly human by day 100 (c). By 200 days after gestation (e), at the very time brain activity begins, the various sections of the brain are recognizable. When the fetus is full term (266 days) (f), all the parts of the brain, including the cortex (the outer layers), are formed, folding over one another and becoming more convoluted, or wrinkled, as the number of brain cells increases.