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Fig. 2 (A-F) The expression pattern of zebrafish Nkx2.5, revealed by whole-mount in situ hybridization, correlates with the position of cardiac precursors. (A) The Nkx2.5-expressing cells (arrows) are located on the left and the right of the midline in part of the lateral plate, at the 10-somite stage. (B) Transverse section reveals that the ,em>Nkx2.5-expressing cells organize in tubes on the right and the left of the midline (arrows) ventral to the neural tube at the 18-somite stage. (C) Starting at 20-somite stage, the two tubes move toward the midline and begin to fuse at the caudal side (arrows). (D) The fusion of the primitive heart tube is complete by the 24-somite stage, shown as a ring-like structure of zebrafish Nkx2.5-expressing cells at the midline (arrow). (E) By 26 hours postfertilization (hpf), the heart moves to the left side of the embryo (arrow). (F) Nkx2.5 expression is restricted to the heart in a 48 hpf embryo (arrow). Anterior to the left in all but B. A, C, D are dorsal views. E is dorsolateral view from the left side; F is left lateral view. hd, head; nt, neural tube.