The brain of a premature fetus, shown here (front of brain pointing to the left) is initially smooth in the early stages of development. After about 20 weeks gestation, grooves develop in the cortical surface that gradually become more defined until they form well delineated gyri and sulci (i.e. bumps and grooves) typical of a mature brain. The brain shown here has a single indentation marking the early development of the lateral sulcus, also known as the Sylvian Fissure.
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