develops from OBO_REL:part_of (UNDECLARED) cardiovascular system heart primordium Bilateral groups of cells consisting of three rows: one row of endocardial precursors medially and two rows of myocardical precursors laterally. The two populations fuse at the midline between 21 and 26 somites to form the heart rudiment or cone. Stainier 2001. TAO:0000028 teleost_anatomy lateral mesoderm