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Fig. 3 Cross-sections of slow and fast myosin heavy chain expression. a,f: Expression of smyhc1(a) and myhc4 (f) is seen in the adaxial cells at the 10-somite stage. b-d,g-i: Single confocal images of fast red heavy chain in situs and F59 antibody labeling at the 26-somite stage. b-d: smyhc1 expression is restricted to the slow muscle as demonstrated by smyhc1 in situ (b) and F59 antibody labeling (c) overlapping (d). e: A 28-somite embryo at the level of the yolk extension showing expression of smyhc1 in the single layer of superficial slow muscle (arrowheads) and the medial muscle pioneer cells (arrows). g-i: The restriction of myhc4 to the fast muscle after slow muscle migration is demonstrated by the nonoverlapping patterns of myhc4 (g) with F59 (h) seen in i. j: Expression of myhc4 is visible throughout the fast muscle but not in the single layer of superficial slow muscle (arrowheads) or the muscle pioneer cells (arrows).