Foramen Primum

**Semantic type:** Embryonic Structure

**Definition:** A temporary passageway enabling oxygenated blood to pass from the right to the left atrium in the embryonic heart. An opening between the free edge of the septum primum and the AV cushions that allows blood to flow directly between the right and left atrium, and which is closed by the union of the septum primum with the septum intermedium.