Fin box morphogenesis. The fin box mesothelium develops from the dorsal-most cells of the external cell layer. (A, B) Dorsal cells from a pair of somites extend over the neural tube (black arrowheads mark the boundary of external cell layer cells). (C) By mid larval stages, a continuous epithelium is present overlying the neural tube. (D) A double-layered fin box mesothelium (arrowhead) is apparent after it separates from the external cell layer (arrowhead marks the dorsal boundary between the external cell layer and myotome). (E, F) Expanded fin box coelom in late larval and metamorphic stages; the mesothelium completely encloses the coelom as shown in transverse (E) and sagittal (F) views. (G) The adult fin box mesothelium is single layered (arrow), and extracellular matrix also accumulates within the dorsal-most dermis separating the coelom and epidermis. White arrowhead indicates a cutaneous canal, which is also shown in detail in (H, I). Scale bars are 2 μm. Stage abbreviations: GS, gill slit; met, early metamorphosis, adult, subadult.