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The process by which the vertebrate neural tube is formed. The primordium of the central nervous system is the neural plate, which arises at the close of gastrulation by inductive action of the chorda-mesoderm on the overlying ectoderm. The axial mesodermal substratum causes the neural ectoderm to thicken into a distinct plate across the dorsal midline and influences both its size and shape. Its shieldlike appearance, broader anteriorly and narrower posteriorly, presages the areas of brain and spinal cord, respectively. The lateral edges of the neural plate then rise as neural folds which meet first at the level of the future midbrain, above the dorsal midline, then fuse anteriorly and posteriorly to form the neural tube. The body ectoderm becomes confluent above the closing neural tube and separates from it. Upon closure, the cells (known as neural crest cells) which occupied the crest of the neural folds leave the roof of the tube and migrate through the mesenchyme to all parts of the embryo, forming diverse structures. The neural tube thus formed gives rise to the brain and about half of the spinal cord. The remainder of the neural tube is added by the tail bud, which proliferates a solid nerve cord that secondarily hollows into a tube. See Nervous system (vertebrate), Neural crest
the embryonic process in chordates and man by which the neural plate emerges and folds to form the neural tube. The embryo undergoing neurulation is said to be in the neurula stage. During neurulation the rudiments of all the organ systems can be discerned in three germ layers.
The external germ layer, or ectoderm, thickens on the dorsal side of the embryo and forms the neural plate; the neural folds arise along the edges of the neural plate. The center of the neural plate deepens, and the crests of the neural folds approach each other and coalesce to form the neural tube, the rudiment of the nervous system. The remaining portion of the ectoderm closes over the neural tube and is transformed into an epithelial covering.
In animals in which the fertilized ovum undergoes total, or holoblastic, cleavage, the innermost germ layer, or endoderm, grows toward the dorsal side of the embryo and completely surrounds the gastrocoele, which thus becomes the primitive digestive cavity of the embryo. In animals with partial, or meroblastic, cleavage of the ovum, the ventral side of the intestine remains open; the intact yolk serves as the intestine’s lower wall.
The middle germ layer, or mesoderm, gives rise to (1) a median, longitudinal, rod-shaped strand of cells—the rudimentary notochord; (2) the somites, or mesodermal segments, that lie on either side of the rudimentary notochord; (3) the nephrotomes, or intermediate cell masses—segmental stalks that extend from the somites; and (4) the lateral plates. By the end of neurulation, the embryo acquires the structural plan of the adult organism: under the epithelium on the dorsal side is the neural tube; under the neural tube, the notochord; and under the notochord, the intestine. The anterior and the posterior segments of the embryonic body are distinguishable.