During the first half of gestation, the human fetal brain has a smooth surface. At about 20 weeks gestation, the brain begins to form the bumps (gyri) and grooves (sulci) that are typical of the adult brain, averaging about 1 sulcus per week of gestation after 20 weeks. The brain of this 25 week gestation fetus shows a gyral pattern that is normal for this gestational age, in which about 4 to 5 sulci can be seen. Note that the first two sulci that develop are the lateral sulcus, which separates the frontal and temporal lobes, and the central sulcus, which separates the frontal and parietal lobes; these are well formed and easily identifiable at this stage.
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