All chordates are distinguished by a further stage of development. A portion of the mesoderm differentiates early and forms a stiff cord running along the length of the body, the notochord (seen in us in embryos, and remaining in the adults as the intervertebral discs). The line of ectoderm along the top of the animal infolds and then buds off along its length, making a hollow tube of ectodermal cells that will go on to form the nerve chord.