Development of the non-myotome somite (1): embryos and early larval stages. Transmission electron micrographs show the position of the non-myotome cells (external cell layer and sclerotome) relative to myotome at the stages indicated. (A-D) are whole somite views showing the progressive change in the position of the sclerotome. In these and all panels, white arrowheads mark the border between sclerotome and the myotome and white asterisks mark sclerotome or external cell layer nuclei. Black arrowheads mark the transition between dorsal myotome and the external cell layer and black arrows indicate the somitocoel (G-I) are details of panels (B-D), respectively, focusing in the position of the sclerotome-myotome boundary. White arrow in (G) indicates muscle fibers. (E, F) are details of the dorsal myotome-external cell layer boundary. Scale bars are 2 μm, dorsal is up, medial is to the left. Structures are labeled on the first panel only: end, endoderm; epi, epidermis; no, notochord; nt, neural tube. Stage abbreviations: neur, neurula; GS, gill slit.