part of develops from start stage end stage cardiovascular system heart primordium TAO:0000028 ZFA:0000028 heart primordia Bilateral groups of cells consisting of three rows: one row of endocardial precursors medially and two rows of myocardial precursors laterally. The two populations fuse at the midline between 21 and 26 somites to form the heart rudiment or cone. zebrafish_anatomy cardiac field lateral mesoderm portion of tissue segmentation:5-9 somites segmentation:20-25 somites