How does the notochord of a cephalochordate differ from the embryonic notochord of most vertebrates?

The notochord of a cephalochordate differs from the embryonic notochord of most vertebrates primarily in its persistence and structural composition.

In cephalochordates, such as amphioxus, the notochord is a permanent structure that extends the entire length of the body throughout the animal's life and is never replaced by a vertebral column. Structurally, it consists of a firm sheath enclosing a core of cells that contain muscle fibers, which help maintain the stiffness required for swimming.123

In contrast, while the embryonic notochord of most vertebrates serves as an initial supporting structure, it is a temporary feature. During development, it is typically replaced or incorporated into the vertebral column as the centers of intervertebral discs, known as the nuclei pulposi. In higher vertebrates, this process involves the surrounding mesodermal tissue (sclerotome) condensing to form vertebrae, which eventually compress or replace the notochordal tissue.452