The notochord is a flexible rod-like structure found in the embryos of all chordates, which include animals like humans, fish, and birds. It serves as a primary support structure during early development and is crucial for the proper formation of the spine. In many vertebrates, the notochord is eventually replaced by the vertebral column or backbone, but it plays a vital role in signaling and organizing surrounding tissues during development. The presence of a notochord is one of the defining characteristics of the phylum Chordata.