"At 8 weeks, the fetal brain is profoundly immature and its surface layer, the cerebral cortex, is smooth, with no indication of the folds (sulci and gyri) that are so prominent later. There is also no internal cellular organisation in either the thalamus, which is the main source of sensory input to the cortex, or the cortex itself. The limbic system, an evolutionary older part of the brain, consisting of interconnected deep brain structures involved in various fundamental drives and regulatory functions, is already discernable and has began to form interconnections. The external surface of the brain is about 1 mm thick and consists of an inner and outer layer with no cortical plate, the structure that will gradually develop into the layers of the cortex proper."