The endocardial progenitor cells are absent from the heart region in clo mutant embryos at the time of heart tube formation. Horizontal sections of a wild-type (A,B) and a clo mutant embryo (C,D) at the 24-somite stage. In wild type, tropomyosin immunoreactive myocardial cells (arrows) surround the endocardial progenitor cells as the two primitive myocardial tubes fuse to form the definitive heart tube (Stainier et al., 1993). In clo, tropomyosin immunoreactive myocardial cells (arrows) surround a space which is conspicuously empty and somewhat collapsed. (Serial sections were examined throughout this region in several mutant embryos and no endocardial progenitor cells were seen). In C and D, a more apical region of the heart cone (i.e. a more arterial region of the heart) is shown which explains why the mutant heart appears smaller than wild type. Scale bar, 50 μm.