Practice question

During vertebrate embryogenesis, notochord serves as axial scaffold and midline signaling organizer secreting Sonic hedgehog and chordin, patterning neural tube and somites. Paraxial mesoderm sclerotome migrates around notochord and neural tube, forming cartilaginous vertebrae that later ossify into vertebral column. Notochord regresses largely, vestiges remaining as nucleus pulposus inside intervertebral discs. Thus vertebral bodies replace notochordal support providing rigid yet flexible axis for muscle attachment, weight bearing and protection of spinal cord. Cartilage alone, exoskeleton or skull do not fully substitute notochord, making vertebral column its adult replacement.