The embryo forms a disk of three layers: endoderm, mesoderm, and ectoderm. The central portion of the ectoderm differentiates into the neural plate, which is made up of neural stem cells. The neural plate forms a groove, called the neural groove, and then rolls up into a tube, called the neural tube. Formation of the neural tube is also called neurulation. Closure of the neural tube starts in the middle and finishes at the ends. On the rising edges of the neural groove, which is the part that will fuse together to close the neural tube, other cells differentiate into what are called neural crest cells. Neural crest cells migrate away from the neural tube and form many tissues of the body, including most of the sensory neurons and the glia of the peripheral nervous system, as well as some of the autonomic neurons. Lower motor neurons and some autonomic neurons in the neural tube, derived from neural stem cells, extend axons out into the nerves to complete many of the nerves of the peripheral nervous system.