Embryological development of the human veebra is form

The paraxial mesoderm becomes segmented to form 40-45 pairs of somites that lie on either side of the developing neural tube and notochord. The somites appear between the 20th and 30th days of development. Hence, the 4th week of development is known as somite period of development. The veebral column is formed from the sclerotomes of the somites. The cells of each sclerotome get conveed into loose mesenchyme. This mesenchyme migrates medially and surrounds the notochord. REF: Inderbir Singh's Human Embryology, eleventh edition, pg. no., 138, 139.