Development of the myosepta. In all panels, myosepta are shown forming between adjacent myomeres. Myosepta are shown in frontal section (A), in sagittal section (F, H) or in transverse section (remaining panels). (A) Extracellular matrix is evident in the position where myosepta form by mid-neurula stages, and (B) a double basal lamina is present in late neurulae. (C) In early larvae, anchors (white arrowhead) that attach muscle cells to the myosepta are first apparent. (D) A complete larval myoseptum is shown, attaching to the notochordal sheath (left) and the dermis (right); nucleus of a sclerotome-derived mesothelial cell is marked with a white asterisk. (E-F) Collagen accumulates within the myosepta during larval and metamorphic stages, although it does not appear striated. In sagittal section (F), the specialized ends of the muscle cells where they terminate on myosepta, are evident. White arrowhead indicates an anchor extending into the muscle cell. (G-H) The myosepta are further thickened in subadults, shown in transverse and sagittal sections, respectively. Black arrowhead in (G) indicates the dermis, white asterisk in (H) indicates the nucleus of a myoseptal cell that appears late in development along the myosepta and is not a muscle cell (see text). All images are to the scale shown at bottom left except (H) where scale bar is 2 μm. Stage abbreviations: neur, neurula; GS, gill slit; post-met, postmetamorphic juvenile; adult, subadult.