Dorsal nerve cord
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The dorsal nerve cord is one of the embryonic features unique to chordates. The dorsal nerve cord is a hollow cord dorsal to the notochord. It is formed from a part of the ectoderm that rolls, forming the hollow tube, compared to other animal phyla, which have solid, ventral tubes. The dorsal nerve cord is later modified in vertebrates into the central nervous system which is composed of the brain and spinal cord.
Dorsal means the "back" side, as opposed to Ventral which is the "belly" side of an organism. In bipedal organisms dorsal is the back and ventral is the front. In organisms which walk on four limbs the dorsal surface is the top (back) and the ventral surface is the bottom (belly).