The cardiac conotruncus has been cut away. The aortic sac (dark green) and the two horns of the aortic sac (light green) remain. At this stage two aortic arches are present (orange = 1st aortic arch, ocher = 2nd aortic arch) that direct the blood from the outflow tract via the pharyngeal arches into the two dorsal aortas (red).

The 3rd aortic arch (violet) appears already one day later. The 1st aortic arch has already atrophied.

With the appearance of the 4th aortic arch (yellow) the 1st aortic arch has disappeared and the 2nd has also begun to atrophy. What remains is only the ventral attachments of the two horns of the aortic sac (light green) that will be involved in the perfusion of the head (internal and external carotid arteries).

The 2nd aortic arch has also now disappeared. One assumes that there is no 5th aortic arch. At the most, fine vessel sproutings are described in the literature that, though, do not form a real 5th aortic arch. They atrophy again very quickly.

In the further development new buddings of the dorsal aorta appear. On the left side (on the right in the diagram) it will later form the ductus arteriosus. Later, they will atrophy again on the right. On the other side, fine vessels bud from the aortic sac that surround the two pulmonary anlagen and later form the pulmonary artery on both sides. As a whole, they form the 6th aortic arch (light yellow) that is also designated the pulmonary arch.

The dorsal buddings from both dorsal aorta soon obtain an attachment to the ventral one. Now the true 6th aortic arch (light yellow) has been formed.

From the dorsal aorta a whole series of intersegmental arteries spread out and form anastomosis among one another, out of which the vertebral artery will arise. Here only the 6th intersegmental artery is shown. Later, the subclavian artery will arise out of it on the left and right because, due to various transformational processes (among others, the descent of the heart), these vessels are pushed upwards higher and higher until they reach the level of the 4th aortic arch.

In the upper region, various vessel segments atrophy, move or reform newly. The aorta dorsalis segment between the 3rd and 4th aortic arches atrophies on the left and the right. The connection to the dorsal aorta in the cranial region remains in place over the third aortic arch. The internal carotid artery will form itself from it. Newly a vessel buds out of the two horns of aortic sac, the external carotid artery (pink).

A further segment, the right dorsal portion of the 6th aortic arch or pulmonary arch atrophies. The left one remains, however, and is vital for the prenatal period because most of the blood from the pulmonary circulation system is shunted via this ductus arteriosus into the systemic one. The ventral portions of the 6th aortic arch remain on both sides and form the pulmonary arteries. The 6th intersegmental artery shifts further upwards on both sides.

The right dorsal portion of the 6th aortic arch (on the left in the diagram) has now wholly disappeared. In the upper part the common carotid artery (light green = horns of aortic sac) with the two descending branches, the internal carotid artery with parts from the 3rd aortic arch and dorsal aorta and external carotid artery are clearly visible as new vessels buds. One can also see that the outflow tract has divided completely into the truncus pulmonalis and the aorta. Responsible for this is the aorto-pulmonary septum that has formed in the outflow tract.

In the further development, further vessel segments atrophy. Here the right aorta dorsalis in the space between the 4th aortic arch and the fusion of the left and right aorta dorsalis is in the process of atrophying. This has a clear relationship with the movement of the heart to the left. The 6th intersegmental artery has reached the level of the 4th aortic arch on both sides.

The vessel segment of the right aorta dorsalis is obliterated. Only a small remnant (red) remains that forms the right subclavian artery with the 4th aortic arch (yellow) on the right and the 6th intersegmental artery (green).

At the end of the embryonic period the vessels of the arteries near the heart resemble those of the adult, with the exception of the ductus arteriosus. Normally, after birth, this will atrophy.
The truncus brachiocephalicus forms from the right horn of the aortic sac while the left common carotid artery forms from the left one.
The left subclavian artery has formed from the left 6th intersegmental artery.