Development of the nervous system.

The neural tube is developed from an infolding of the ectoderm along the dorsum of the embryo, from which cells detach to form the neural crests on each side. From neural crests are developed the craniospinal ganglia and perhaps also the autonomic ganglia. The cells of the neural tube, the wall of which at first is composed of a single layer of epithelium, undergo rapid division and differentiate into spongioblasts, which later form neuroglia. Ependima develops from the primitive lining of the neural tube. In the neural crests, differentiation occurs and neurons, satellite cells, and neurolemma are formed. Microglia probably are formed by mesenchymal cells, which enter the central nervous system with the blood vessels.

The central nervous system appears in the middle of the third week of the development as a thickened area of the embryonic ectoderm, the neural plate. Its lateral edges become elevated to form the neural folds, which approach each other and fuse in the middle, thus forming the neural tube.

At the cranial and caudal end of the embryo the neural tube is temporarily open and communicates with the amniotic cavity by the way of the cranial and caudal neuropores. The neural tube differentiates into the central nervous system, consisting of the brain and spinal cord, and the neural crest, which gives rise to the most of the peripheral nervous system. The neural canal becomes the ventricular system of the brain and the central canal of the spinal cord.

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