- Developing heart forms constrictions --> bulbus cordis, ventricle and atria.
Primitive atrium is still paired and connected caudally to the paired sinus venosus.
Heart tube bends ventrally, caudally and slightly to the right
During looping --> anterior-posterior embryonic polarity changes to right-left polarity seen in adult
Paired sinus venosus extend laterally --> sinus horns.
Paired atria form a common chamber and move into the pericardial sac.
Partitioning of atrium from ventricle is due to the secretion of cardiac jelly --> form endocardial cushion