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Foetal brain development, artwork. A fully developed brain is at right. During the 4th week (25 days) the neural tube begins to differentiate into a spinal cord, forebrain, midbrain and hindbrain. During the 6th week (40 days) the forebrain differentiates into the telencephalon and diencephalon. The telencephalon develops into the hippocampus, basal ganglia, amygdala and the cerebral cortex. The diencephalon develops into the thalamus, hypothalamus and hypophysis. (KEYSTONE/SCIENCE PHOTO LIBRARY/FRANCIS LEROY, BIOCOSMOS)