In the space directly around the notochord (axial perinotochordal space, perinotochordal sheath) the sclerotome cells remain unsegmented and loosely packed. However, in the lateral or paraxial space they form segments (sclerotomes) at S13. By S14, a loose cranial and a dense caudal half appear in the sclerotomes. The loose cranial part is traversed by intersegmental arteries and nerves. The neural processes and the costal processes are formed from the dense caudal part.