part_of part of part of develops from develops from ends during starts during cardiovascular system heart primordium The fused aspects of ventral mesoderm, which have migrated from either side of the prechordal plate, and fused ventrally, just behind the cement gland. They give rise to the endocardium at NF stage 27/28. fused heart primordium XAO:0000336 xenopus_anatomy UBERON:0003084 heart primordium primary heart field secondary heart field NF stage 19 NF stage 31