Human Embryology and Teratology
Teaching text 15: Axial Skeleton 3: Vertebral column (S15)
By S15, loose and dense zones also appear in the perinotochordal space. The dense zones lie just above the middle of a segment. They are the precursors to the perinotochordal discs and then the intervertebral discs. They delimit the loose zones, or centra. These are the precursors of the vertebral centres, and later develop to comprise the bulk of the vertebral body.