The notochord is a flexible, rod-shaped body found in embryos of all chordates. It forms the axis of the embryo. In some chordates, it persists throughout life as the main support down the length of the body. In most vertebrates it is replaced by the vertebral column. The notochord is found ventral (underneath) the neural tube (future spinal chord).
Notochords were the first "backbones" serving as support structures in chordates. Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment, yet flexible enough to allow movement. Embryos of vertebrates have notochords today, as embryonic development often follows a pattern similar to the ancestral evolution of the modern animal's traits.