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Fig. S4 The number of cmlc2-expressing cells increases gradually over time. (A-D) In situ hybridization for cmlc2 in wild-type embryos; dorsal views, anterior to the top. (A) At the 14-somite stage, only a few cells have initiated cmlc2 expression. (B) By the 16-somite stage, more cmlc2-expressing cells are detectable. These cells appear to be located near the medial edge of the lateral plate mesoderm, which is migrating towards the embryonic midline. (C) More cmlc2-expressing cells are evident by the 18-somite stage. As the bilateral cardiomyocyte populations prepare to fuse, cmlc2 expression appears to be spreading laterally within the lateral plate mesoderm. (D) Continued increase in the lateral expansion of cmlc2-expressing cells is apparent at the 20-somite stage, as cardiac fusion begins. The medial-to-lateral spread of cmlc2 expression within the lateral plate mesoderm suggests that cmlc2 expression in ventricular cells precedes cmlc2 expression in atrial cells, as ventricular precursors are located in the medial portion of the lateral plate mesoderm and atrial precursors are located more laterally (Berdougo et al., 2003; Glickman and Yelon, 2002; Schoenebeck et al., 2007)