Step-by-step explanation:
In most developing vertebrates the notochord is a transient structure that plays a critical role in the early development of the embryo. The notochord is a rod-like structure that runs along the dorsal side of the developing embryo and provides support and signaling cues for the developing nervous system.
As the embryo develops the notochord begins to be replaced by the vertebral column which is made up of individual vertebrae that surround and protect the spinal cord. The notochord is gradually replaced by the centrum of each vertebra which forms around the notochord and eventually fuses with adjacent vertebrae to form the complete vertebral column.
In some vertebrates such as lampreys and hagfish the notochord persists throughout the animal's life and serves as the main axial support structure. However in most vertebrates the notochord is only present during embryonic development and is replaced by the vertebral column as the animal matures.