The<unk>  is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In lower vertebrates, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column. The<unk>  is found on the ventral surface of the neural tube.<unk> s were the first "backbones", as well, serving as support structures in chordates that lacked a bony skeleton. The very first vertebrates, such as Haikouicthys, had only a<unk> . Embryos of vertebrates have<unk> s today, as embryonic development often happens to follow a pattern similar to the ancestral evolution of the modern animal's traits [this idea that 'ontogeny recapitulates phylogeny' is something of an old-fashioned concept in embryology].<unk> s were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment, yet flexible enough to allow more movement than, for example, the exoskeleton of the dominant animals of that time. In humans, they eventually develop into the disks between the vertebrae.
Notochord
Yolk sac
Branchial pouch
Osteotome
Neural crest
Growth cone
Branchial arch
Dura mater