From S17, the caudal dense sclerotomic areas make contact with the cranial zone of the vertebral centra in the perinotochordal space. At the same time, the neural processes extend dorsally and surround the spinal cord. The notochord undergoes segmentation in the perinotochordal space. It expands in the region of the intervertebral discs and becomes the nucleus pulposus. However, it thins out in the region of the vertebral body.