
The subsequent main phases of CNS development involve cell proliferation, migration, and differentiation together with synaptogenesis and apoptosis and finally myelination. Neural crest cells form the peripheral nervous system. Some neuroectodermal cells remain between the neural tube and surface ectoderm as the neural crest. This strip of neuroectoderm folds in to form the neural groove with neural folds on either side which roll up and fuse to form the neural tube during the 4th week. The neural plate is induced by signals from the notochord. The nervous system develops from the neural plate, a dorsal thickening of ectoderm, which appears on the dorsal surface of the early embryo in the 3rd week of gestation.

The nervous system consists of three parts:
