Fig. 1. Fibre typing and tracing of muscle fibre outlines in cross sections. (A) 40-somite (stage-1) embryo of the warm regime immunoreacted with the F59 antibody. The superficial monolayer of red fibres (RF) stains more intensively than the bulk of white fibres (WF) underneath. Scale bar, 100 µm. (B) Stage-5 larva of the cold regime. Detail of an epaxial quadrant with fibre contours and myofibrils as visualized by PAMS. Myofibrils form garland-like patterns in red fibres (RF) but radial patterns in white fibres (WF). Scale bar, 25 µm. (C,D) Discrimination between fibre types in the horizontal septum area. (C) Stage-6 larva (warm regime). Histochemical staining for myofibrillar ATPase after preincubation at pH 10.2 leaves white fibres (WF) active while red fibres (RF) are inactivated. Scale bar, 50 µm. (D) Swim-up larva (stage 3, cold regime). Immunostaining with the S58 antibody highlights the red fibres (RF) monolayer, white fibres (WF) are unreactive. Scale bar, 25 µm. spc, spinal cord; nc notochord; ms, myoseptum; hs, horizontal septum; ln, lateral line nerve; ep, epidermis.

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