Final answer:
The smooth part of the left atrium is derived from the primitive atrium, specifically the anterior portions of both the right and left atria and the two auricles.
Step-by-step explanation:
The smooth part of the left atrium is derived from the primitive atrium, specifically the anterior portions of both the right and left atria and the two auricles. The left atrium does not contain pectinate muscles, but it does have an auricle that includes pectinate ridges.