| 1 | First Functional Organ | The heart is the first organ to function, starting to beat by day 22 post-fertilization. |
| 2 | Origin Layer | The heart develops from splanchnic mesoderm in the cardiogenic area. |
| 3 | Cardiac Tube Formation | Begins in week 3 of gestation as paired heart tubes that fuse into a single tube. |
| 4 | Cardiac Looping | Occurs during week 4, positioning chambers correctly (D-looping normally). |
| 5 | Truncus Arteriosus | Gives rise to ascending aorta and pulmonary trunk. |
| 6 | Bulbus Cordis | Forms the smooth parts of right and left ventricles (outflow tracts). |
| 7 | Primitive Ventricle | Forms most of the left ventricle. |
| 8 | Primitive Atrium | Forms the trabeculated parts of both atria. |
| 9 | Sinus Venosus (Right Horn) | Forms the smooth part of right atrium (sinus venarum). |
| 10 | Sinus Venosus (Left Horn) | Forms the coronary sinus. |
| 11 | Septum Primum & Septum Secundum | Contribute to the interatrial septum and formation of foramen ovale. |
| 12 | Endocardial Cushions | Participate in forming AV canals, AV valves, and parts of septa. |
| 13 | Neural Crest Cells | Crucial for aorticopulmonary septum, separating aorta and pulmonary trunk. |
| 14 | Aorticopulmonary Septum | Spirals to divide the truncus arteriosus into great vessels. |
| 15 | Interventricular Septum Formation | Formed by muscular septum, membranous septum (from cushions + conotruncal ridges). |
| 16 | AV Valve Development | Derived from endocardial cushion tissue through remodeling. |
| 17 | Epicardium Origin | Arises from proepicardial organ, part of splanchnic mesoderm. |
| 18 | SA Node Development | Derived from right common cardinal vein and right sinus venosus. |
| 19 | Pulmonary Vein Formation | Develops as an outgrowth from left atrium, connecting to lung buds. |
| 20 | Dextrocardia | Caused by abnormal (leftward) looping of the heart tube. |
