Division of the Truncus Arteriosus
By the 28th day of development, the 
truncus
arteriosus divides by means of a bisecting spur which protrudes from
both the aorta and pulmonary artery. The division continues in a downward
direction until the two vessels are completely separated giving rise to
the 
aortic
and pulmonary channels . This division begins at the distal end and
progresses toward the conus cordis. At the level of the conus cordis, the 
truncal
swellings develop into pulmonary and aortic valve cusps and form two
of
the three cusps in each valve. A third swelling,
the intercalated swelling, forms the third cusp.
truncus
arteriosus divides by means of a bisecting spur which protrudes from
both the aorta and pulmonary artery. The division continues in a downward
direction until the two vessels are completely separated giving rise to
the 
aortic
and pulmonary channels . This division begins at the distal end and
progresses toward the conus cordis. At the level of the conus cordis, the 
truncal
swellings develop into pulmonary and aortic valve cusps and form two
of
the three cusps in each valve. A third swelling,
the intercalated swelling, forms the third cusp.
