Coronal scheme demonstrating the formation of the vertebral column. A) Sclerotome cells migrate towards the midline to form the perichordal zone. B) Cranial (low cellular density) and caudal (high cellular density) areas form the sclerotomes. The perichordal zone is still unsegmented. C) Dense and loose areas develop in the perichordal zone. D) The dense areas in the perichordal zone develop into intervertebral discs and the loose areas develop into vertebral centra.