Proximal pa / arch of aoa is fromed by: 1. Left horn of aoic sac 2. Left 4th phyrangeal arch aery 3. Left dorsal aoa - Neural crest cells migrate to truncus aeriousus and forms AP septum (spiral septum) which divides Truncus into anterior Aoa and posterior Pulmonary trunk. Thus the proximal pa of the aoa is derived from the truncus aeriosus. - From truncus aeriosus develops aoic sac with left and right horns . - Truncus aeriosus forms the outflow tract and blood is pumped by hea into phrangeal arch aeries and dorsal aoa