Hearts were imaged using embryos from the myl7:dsRed-nuc reporter line and numbers of cardiomyocytes counted manually in Z-stack sections. The top panel (A) shows the quantification of data for wildtype (WT) or double morphant embryos (Mo) at 2 dpf or 3 dpf as indicated. Also as indicated, the cells were counted in the heart tube (HT) at 2 dpf or specifically in the ventricle (V) or atrium (A) at 3 dpf. The ** indicates statistical significance compared to wildtype, according to Student’s t-test, p<0.01. Lower panels show representative images of wildtype (WT) or tbx2ab morphants (MO) at 2 dpf (B, D) and 3 dpf (C, E). For each measurement n is at least 10.