The neural system develops from ectodermal cells, the outermost layer of embryonic germ cells. The covering of the neural system and the blood vessels that supply nervous tissue are mesodermal in origin.

Immediately after the formation of the embryonic disk of cells, there is a thickening in the ectoderm along the longitudinal axis to form the neural plate. The plate then folds along its long axis to form a groove. The sides of the groove are neural folds that fuse on the dorsal side to form a neural tube. A number of cells termed neural crest cells are pinched off the top of the primitive neural tube as it separates from the overlying ectodermal layer, The neural tube is divided into three basic layers, and the cell types within these layers take on the layer name. The outermost layer of cells in...