A notochord can be found in the embryos of all chordates, the phylum which contains the subphyla of fish, mammals, reptiles, birds and amphibians. The notochord defines the primitive axis of the embryo, is a transient embryonic anatomy structure and is not usually found in adults.
In most vertebrates, the notochord transforms into different structures during the gestational period. Some creatures such as lampreys, hagfish and tadpoles retain a post-embryonic notochord. In the human embryo, the notochord forms in the third week of pregnancy. It is required to pattern the folding of the embryo, and eventually plays a role in the growth of the nucleus pulposus and intervertebral disc.
A condition called Tornwaldt’s cyst can occur when remnants of the notochord persist along the human vertebrate. The cysts are typically small and asymptomatic, and found incidentally through an MRI necessary for other medical needs. A large cyst will typically present with symptoms of obstruction.
Study of the notochord has helped researchers in understanding the embryonic development of the central nervous system. Specifically, the notochord is responsible for secreting the signaling protein, "Sonic Hedgehog" (SHH, Shh), which has a critical role in signaling the development of motor neurons and the formation of digits and limbs.