Fig. 1. Bmp signaling and the regulation of cardiogenesis in the early chick embryo (stage 5-6). The embryo has been divided into three zones, from medial to lateral. Bmp ligands (blue) are expressed in zones II and III, in both anterior and posterior regions of the embryo (Schultheiss et al., 1997). Heart precursors are found in a subportion of the Bmp-expressing zone (zone II) in the anterior part of the embryo. More medial regions of the embryo (zone I) do not express Bmp2 or Bmp4, and do not express cardiac genes. More lateral regions (zone III) are exposed to Bmp signals yet do not express cardiac genes.