During the third week of development the outermost layer of the embryo—the ectoderm—thickens to form a neural plate (eFig. 9.2A). This plate develops a longitudinally running neural groove, which deepens so that it is flanked on either side by neural folds (eFig. 9.2B). These folds further develop and eventually fuse during a process called neurulation to form a long tubelike structure called the neural tube with an inner lumen called the neural canal (eFig. 9.3). Fusion of the tube starts at the midpoint and extends cranially and caudally so that the tube is fully formed by the fourth week. Continued proliferation of the cells at the cephalic end cause the neural tube to dilate and form the three primary brain vesicles (eFig. 9.4): the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain), which later give rise to the structures of the brain. Caudally, the neural tube lengthens and narrows to form the spinal cord. The neural canal forms the cavities of the ventricular system in the brain and central canal of the spinal cord (eTable 9.1). The peripheral nervous system consists of cranial nerves, spinal nerves, spinal ganglia, the enteric system, and autonomic ganglia. The peripheral nervous system is formed by nerve fibers that extend out of the central nervous system and by neurons and their fibers that develop from migratory neural crest cells (eFig. 9.4A). Like the neural tube, neural crest cells originate from surface ectoderm and initially lie on each side of the developing CNS.