(A, B) Morphology of the developing cucullaris, with the four gill-levator muscles (lab I–IV) shaded light green and the cucullaris (cuc) blue. More-posterior muscles are shaded dark green. Anterior is to the left. (A) Dorsolateral view of an OPT scan of a juvenile axolotl stained with the 12/101 muscle antibody. (B) Contrast-stained CT scan of an adult axolotl in lateral view. The cucullaris is expanded into a broad sheet that inserts on the scapula. (C) Schematic depiction of an orthotopic transplantation of unsegmented mesoderm lateral to somites 1−3 at stage 21. Lateral views; anterior is to the left. nc, neural crest; nt, neural tube; s3, somite 3. (D−I) GFP labeling following stage-21 transplantation of unsegmented mesoderm lateral to somites 1−3. (D−F) Labeling of the levator arcuum branchiarum anlagen (laba) dorsal to the developing gills is visible in lateral (D, E) and dorsal (F) views. Anterior is to the left. (G) Gill-levator muscles (levator arcuum branchiarum, lab) of arches 3 and 4, the cucullaris (cuc) and the dilatator laryngis (dil) are labeled in a juvenile axolotl. Dorsal view; anterior is to the left. (H, I) Transverse sections through the posterior occipital region (H) and anterior trunk (I) of a juvenile axolotl. GFP labeling is visible in the gill levators and anterior cucullaris (H) and in the posterior cucullaris near its attachment with the scapula (I; sc). Lateral is to the left; dorsal is to the top. (J−M’) isl1 expression in albino embryos. (J) At stage 34, isl1 is expressed in ventral mesoderm, in the developing heart region (arrowheads) and around the dorsal cranial placodes (cp). Arrow indicates several stripes of expression dorsal to the developing gills. (K) At stage 36, isl1 marks the profundal (gPr)/trigeminal (gV) placode region and earlier expression is maintained dorsal to the gills (arrow). (L) At stage 40, isl1 is expressed in neurons within the dorsal spinal cord (arrow) and in the gill-levator region (arrowheads). Dorsal view. (M, M’) Transverse section of a stage-40 embryo with isl1 expression in the dorsal gill levator region (arrows) and ganglia (arrowheads). Box in M is enlarged in M’. Scale bars, 100 μm, except G, 500 μm.