The cardiovascular system is the first system to function in the embryo. In the middle of the third week, the cells of the splanchnic mesoderm proliferate to form the angiogenetic clusters. The ventral portion of a horseshoe-shaped plexus of blood vessels forms the cardiogenic area, while the lateral portions form the dorsal aortae.
Initially, the cardiogenic area is located anterior to the prechordal plate and the neural plate. The growth of the central nervous system pulls the cardiogenic area ventrally and caudally to the buccopharyngeal (oropharyngeal) membrane.
The canalization of cardiogenic clusters in the splanchnic mesoderm results in the formation of the paired heart tubes.

1. Neural plate
2. Primitive node
3. Primitive streak
4. Cut edge of amnion
5. Angiogenetic cell clusters

"These heart tubes approach each other and fuse to form a single heart tube, temporarily attached to the dorsal side of the pericardial cavity by the dorsal mesocardium. The splanchnic mesoderm around the heart tube (future endocardium) thickens and forms the myoepicardial mantle (future myocardium and epicardium).

1. Epimyocardial cells
2. Heart tube
3. Endoderm
4. Endocardial cells
5. Notochord
6. Dorsal aorta
7. Neural crest
8. Neural fold

1. Foregut
2. Intraembryonic coelom
3. Heart tubes
4. Dorsal mesocardium
5. Epimyocardium
6. Neural groove
7. Neural crest
8. Notochord
9. Dorsal aorta