The left panel depicts an early head fold stage. At this stage, the first heart field forms the cardiac crescent (red) while the second heart field (green) lies anterior and medial to the first heart field. The cells of the cardiac crescent join in the midline to form a linear heart tube. The middle panel depicts the heart tube as it begins to loop. Around this time, cells of the second heart field have migrated into the heart tube and will eventually contribute to cells of the atria, outflow tract, and right ventricle. Following rightward looping, cardiac neural crest cells migrate and contribute to the formation of the cardiac outflow tract. The right panel depicts the 4-chambered heart, denoting contributions from the first heart field, second heart field, epicardial progenitors, and cardiac neural crest.