The nervous system develops from embryonic tissue called the ectoderm. The first sign of the developing nervous system is the neural plate that can be seen at about the 16th day of development. Over the next few days, a “trench” is formed in the neural plate - this creates a neural groove. By the 21st day of development, a neural tube is formed when the edges of the neural groove meet. The rostral (front) part of the neural tubes goes on to develop into the brain and the rest of the neural tube develops into the spinal cord. Neural crest cells become the peripheral nervous system.
Foetal brain development, artwork. During the 4th week (25 days) the neural tube begins to differentiate into a spinal cord, forebrain, midbrain and hindbrain. These areas continue to develop over the 5th week (35 days). 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 and hypothalamus. By the 8th week (50 days) the cerebral hemispheres have formed and the brain is growing rapidly. By the 15th week (100 days) the cerebellum and medulla oblongata have formed from the hindbrain.
Foetal brain. Ventral (bottom) view of the brain of a 24-week-old foetus, the front of the brain is at top. The foetus’ developing spinal cord (lower centre) can be seen protruding from the brain. Below the spinal cord is the cerebellum, which controls motor function and maintenance of balance. The small organ at centre is the pituitary gland. This is an endocrine gland whose overall role is to regulate growth and metabolism. At this stage of development the foetal brain shows active auditory and visual responses.