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).

The heart tube elongates and develops dilatations and constrictions: the truncus arteriosus, bulbus cordis, ventricle, atrium and sinus venosus.
The heart tube bends upon itself, forming a U-shaped bulboventricular loop.
The primitive heart tube has only one atrium and one ventricle. Their partitioning begins around the middle of the 4th week and is completed by the end of the 5th week.

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











1. Sinus venosus
2. Truncus arteriosus
3. Bulbus cordis
4. Ventricle
5. Atrium
6. Cardinal veins

Partitioning of the atrium, ventricle, bulbus cordis and truncus arteriosus:
Atrium
At the end of the fourth week, a crest (septum primum) grows from the dorsocranial wall of the primitive atrium towards the endocardial cushions. The temporary opening (ostium primum) between the lower rim of the septum primum and the endocardial cushions gradually closes. Perforations in the upper part of the septum primum form the ostium secundum.
Subsequently, another crest (septum secundum) grows from the right side of the septum primum and gradually covers the foramen secundum. The oval opening left by the septum secundum is called foramen ovale.

1. Venous valves
2. Septum primum
3. Ostium secundum
4. Septum secundum
5. Pulmonary veins
6. Endocardial cushions
7. Interventricular foramen
8. Muscular interventricular septum
9. Foramen ovale

Ventricle
By the end of the 4th week, the two primitive ventricles begin to expand and gradually fuse together, thus forming the muscular interventricular septum. The interventricular foramen found above the muscular interventricular septum subsequently closes by the outgrowth of endocardial cushions which fuse with the caudal part of the aorticopulmonary septum. After the complete closure, the interventricular foramen becomes the membranous interventricular septum.

1. Septum primum
2. Septum secundum
3. Pulmonary veins
4. Ostium secundum
5. Foramen ovale
6. Membranous interventricular septum
7. Muscular interventricular septum
8. Endocardial cushions

Bulbus cordis and truncus arteriosus
During the 5th week, a pair of opposing ridges appear in the walls of the bulbus cordis and truncus arteriosus. These ridges twist around each other, forming a spiral course of the aorticopulmonary septum. This septum divides the bulbus cordis and truncus arteriosus into two channels, the aorta and the pulmonary artery.
It also participates in the closure of the interventricular foramen

1. Aorta
2. Left pulmonary artery
3. Pulmonary trunk
4. Muscular interventricular septum
5. Right ventricle
6. Membranous interventricular septum

1. Aorticopulmonary septum
2. Pulmonary valve
3. Pulmonary artery
4. Aortic valve
5. Aorta

Formation of the sinus venosus, atria and ventricles 
The sinus venosus receives blood from the umbilical, vitelline and common cardinal veins. Initially it is a separate chamber which opens into the caudal wall of the right atrium.
The primitive right atrium enlarges by the incorporation of the right sinus horn (sinus venarum) into its wall. A remnant of the primitive right atrium becomes trabeculated, while the smooth-walled part of right adult atrium originates from the right sinus horn.

1. Superior vena cava
2. Pulmonary veins
3. Left atrium
4. Right atrium
5. Septum primum
6. Septum secundum
7. Primitive left atrium
8. Primitive right atrium
9. Valve of inferior vena cava
10. Valve of coronary sinus
11. Sinus venarum

In the posterior wall of the primitive left atrium develops a pulmonary vein, which progressively branches. During further development, the left atrium expands greatly and incorporates branches of the pulmonary vein (the smoot-walled part of left adult atrium).
The bulbus cordis is gradually incorporated into the walls of the ventricles. It is represented in the right ventricle by the conus arteriosus, while in the left ventricle the bulbus cordis forms the aortic vestibule.

1. Membranous interventricular septum
2. Muscular interventricular septum
3. Endocardial cushions
4. Left ventricle
5. Right ventricle
6. Ventricular wall

Partitioning of the atrioventricular canal
Endocardial cushions develop in the dorsal and ventral walls of the heart in the region of the atrioventricular canal. They grow towards each other and fuse, thus dividing the atrioventricular canal into right and left orifices. In addition, the lateral interventricular cushions appear on the borders of the canal. These localized proliferations of the mesenchymal tissue subsequently differentiate into bicuspid and tricuspid atrioventricular valves.

1. Lateral endocardial cushion
2. Ventral endocardial cushion
3. Dorsal endocardial cushion
4. Left atrioventricular canal
5. Right atrioventricular canal

1. Left atrioventricular canal
2. Right atrioventricular canal
3. Tricuspid valves
4. Bicuspid valves