In order to fit within the confined space of the cranium, the linearity of the brain gives way to a folded structure as seen here. Convolutions develop in the last 12 weeks of development. Midbrain and cervical flexures facilitate development of brain within cranium, convolutions facilitate development of cerebral cortex. The cerebrum contains 85%, and the cortex contains 2/3, of the 100 billion neurons and ¾ of the 100 trillion connections found in the brain. In order to fit within the confined space of the cranium, the linearity of the brain gives way to a folded structure as seen here. Convolutions develop in the last 12 weeks of development. Midbrain and cervical flexures facilitate development of brain within cranium, convolutions facilitate development of cerebral cortex. The cerebrum contains 85%, and the cortex contains 2/3, of the 100 billion neurons and ¾ of the 100 trillion connections found in the brain.