This animated GIF depicts the separation of the common atrioventricular canal into right and left atrioventricular canals. Cardiac jelly forms the endocardial cushions that surround the common AV-canal. These will ultimately form elements of the cardiac skeleton, including the cusps of the atrioventricular and semilunar valves.

Growth and fusion of the anterior and posterior endocardial cushions separates the common AV-canal into right and left AV-canals as the atria and ventricles expand around them.

The right AV-canal connects the right atrium to the right ventricle and will be guarded by the tricuspid valve. The left one connects the left atrium and ventricle and is guarded by the bicuspid (mitral) valve.

Initially, the common atrium opens into a single atrioventricular canal that connects to the common ventricle. Through proliferation and fusion of the endocardial cushions, the atrioventricular canal becomes divided into separate channels, one connecting the right atrium to the right ventricle through the right atrioventricular (tricuspid) orifice and the other connecting the left atrium to the left ventricle through the left atrioventricular (bicuspid) orifice.

Around day 30, partitioning of the atria begins with the development of the septum primum, a thin membrane that grows downward from the roof of the common atrium. As it grows toward the endocardial cushions, it narrows the space between the right and left sides of the common atrium. The narrow connection between the right and left atria, called the osmium primum, will be closed when the septum primum fuses with the endocardial cushions.

Before ostium primum can be obliterated, a new opening develops in the upper end of septum primum. This is called the ostium secundum, or "second opening". This occurs around day 33 of development.

Around day 35, a second membrane begins to grow downward from the roof of the right atrium. This is called the septum secundum, or "second septum" It never reaches the endocardial cushions, leaving a crescent shaped opening called the foramen ovale.

Foramen ovale forms a one way "flutter" valve that shunts blood from the right atrium to the left atrium during development. The purpose of the shunt is to bypass the fetal lungs which are not functional until after birth. In the fetus, oxygenated blood is brought from the placenta into the fetus through the umbilical vein.

This stream of highly oxygenated blood is directed toward the foramen ovale and into the left atrium.

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Blood that enters the right atrium through the IVC is shunted into foramen ovale to the left atrium and left ventricle. It is pumped into the aorta from the left ventricle.

During fetal life, higher pressure in the right atrium favors the function of the foramen ovale as a one way shunt.

At birth, higher pressure shifts to the left atrium and septum primum is pushed over the foramen ovale, closing it.

This becomes the fossa ovalis that can be observed in the interatrial wall of the right atrium.

Note that the right and left ventricles communicate with one another through the interventricular foramen.

While the common atrium is being partitioned by septum primum growing down toward the endocardial cushions, the common ventricle is being divided by the formation of the interventriculat septum.

The muscular part of the inter ventricular septum grows upward from the ventricular wall, narrowing the gap between the inter ventricular septum and the endocardial cushions. The endocardial cushions will contribute to the membranous part of the inter ventricular septum.

Completion of the inter ventricular septum occurs roughly around the same time as the development of the aorticopulmonary septum.

At the beginning of week 7, the atrioventricular canal has been divided into right and left atrioventricular canals, outflow channels from the right and left atria respectively.
At this time the interventricular septum is only partially complete.

Neural crest cells, derived from the region of the rhombencephalon (hindbrain) contribute to the formation a a pair of conotruncal ridges (yellow and green) that will form the spiral aorticopulmonary septum within the truncus arteriosus. 
The conotruncal ridges fuse with each other and the endocardial cushions to divide the truncus arteriosus into two channels:

• pulmonary trunk for outflow from the right ventricle to the lungs
• aorta for outflow from the left ventricle to the body tissues

The red line represents blood flow from left atrium and ventricle into the aorta. The blue line represents blood flow from the right atrium and ventricle into the pulmonary trunk.

Three cross sections through the conotruncal region of the heart demonstrate the spiral nature of the aorticopulmonary septum.

• Initially, the pulmonary outflow (p) is anterior to the aortic outflow (a).
• Distal to the conus arteriosus, the aortic outflow (a) moves to the right of the pulmonary outflow (p)
• Finally, the aortic outflow (a) becomes anterior to the pulmonary outflow (p).

It is the spiral nature of the aorticopulmonary septum that causes the aorta to over arch the pulmonary trunk in the superior mediastinum.