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Fig. 4 F59 intensely labels a monolayer of superficial muscle cells derived from adaxial cells and weakly labels deep muscle cells derived from lateral presomitic cells. Adaxial (A) or lateral presomitic (B) cells were injected at about 14 h with vital dye. At 40 h, embryos were fixed and sections stained with F59. (A) The injected adaxial cell (arrowhead, yellow due to double fluorescence of red and green) became a superficial muscle cell strongly labeled by F59 (green). (B) The injected lateral presomitic cells (arrowhead, red) became deep muscle cells only weakly labeled by F59 (green). In a series of similar experiments, 6 out of 6 injected adaxial cells became part of the superficial layer of strongly F59 reactive muscle fibers; 12 out of 12 injected lateral presomitic cells differentiated into the deeper, weakly F59 reactive muscle fibers. We have also used the monoclonal antibody zn-5, which labels the same population of superficial muscle cells that are strongly labeled by F59 at this time (data not shown). In a series of similar experiments, 18 out of 19 injected adaxial cells differentiated into one of the superficial monolayer of muscle fibers positive for zn-5 (the single exception was the same cell mentioned in the legend to Fig. 2, that was dorsal and may have been adjacent to the neural keel instead of the notochord). In these transverse sections, dorsal is up. Scale bar, 50 μm.