The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoblast and defining 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.
Its appearance synchronizes with that of the neural tube. On the ventral aspect of the neural groove an axial thickening of the endoderm takes place; this thickening assumes the appearance of a furrow (the chordal furrow) the margins of which come into contact, and so convert it into a solid rod of cells (the notochord) which is then separated from the endoderm.
In higher vertebrates, it extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of the mid-brain, where it ends in a hook-like extremity in the region of the future dorsum sellę of the sphenoid bone. It lies at first between the neural tube and the endoderm of the yolk-sac, but soon becomes separated from them by the mesoderm, which grows medial-ward and surrounds it. From the mesoderm surrounding the neural tube and notochord, the skull and vertebral column, and the membranes of the brain and medulla spinalis are developed.
