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Foetal brain development, artwork. During the 4th week (upper left) the neural tube begins to differentiate into a spinal cord (green), forebrain (blue), midbrain (grey) and hindbrain (orange). These areas continue to develop over the 5th week (top). During the 6th week (upper right) the forebrain differentiates into the telencephalon (pink) and diencephalon (yellow). The telencephalon develops into the hippocampus, basal ganglia, amygdala and the cerebral cortex. The diencephalon develops into the thalamus and hypothalamus. By the 8th week (lower right) the cerebral hemispheres (pink) have formed and the brain is growing rapidly. By the 15th week (lower left) the cerebellum (upper orange area) and medulla oblongata (lower orange area) have formed from the hindbrain. (KEYSTONE/SCIENCE PHOTO LIBRARY/FRANCIS LEROY, BIOCOSMOS)