deoxyribonucleic acid, desoxyribonucleic acid, DNA(noun) (biochemistry) a long linear polymer found in the nucleus of a cell and formed from nucleotides and shaped like a double helix; associated with the transmission of genetic information.
But even adults don't often know what a notochord is. A notochord is a primitive beginning to the backbone. It appears in embryos as a small flexible rod made from cells from the mesoderm, which is one of the three layers of cells of embryos. Notochords are only found in the phylum chordata, a group of animals that includes humans.
THE NOTOCHORD. Before there vertebrae, there was a notochord. Notochords evolved in invertebrates known as chordates and there are a few living examples of primitive chordates, such as lancets and tunicates. The most primitive fish today, the jawless fish, do not have full-fledged vertebrae, and still depend on the notochord as their primary axial support. Even once vertebrates began to develop vertebrae, these structures arose in pieces around a notochord which still ran uninterrupted from head to tail.