Development of the notochordal sheath (1): embryonic and larval stages. In all panels, the notochord is to the right and the somitic mesoderm is to the left (in panels (A-E), myotome is adjacent to the notochord, at later stages, in panels (F-J), the sclerotome derived mesothelium is present between the notochord and myotome, see text). The notochordal sheath develops at the interface between them. Particulate extracellular matrix is present at the site of the future notochordal sheath in neurulae (A-B), and a basal lamina associated with the notochord is present by late neurula stages (C). Collagen increases within the sheath during larval and metamorphic stages (D-J). White arrowhead indicates an anchor forming between the notochordal sheath and notochord lamella. Black arrowhead in (F) indicates striated (circumferential) collagen fibers, which are first apparent at mid-larval stages. Black arrow in (J) indicates the outer layer of longitudinal collagen fibers. All images are to scale and all sections are transverse. Stage abbreviations: neur, neurula; GS, gill slit; met, early metamorphosis.