Final answer:
The foramen ovale is the large opening below the growing septum primum in the fetal heart, and the septum primum closes this opening shortly after birth.
Step-by-step explanation:
The large opening below the growing septum primum in the fetal heart is called the foramen ovale. The septum primum is a flap of tissue that functions during fetal development to allow blood to pass directly from the right atrium to the left atrium, bypassing the lungs which are not yet in use. Shortly after birth, as the newborn begins to breathe, the septum primum acts as a valve to close the foramen ovale, which usually fuses to become the fossa ovalis, effectively separating the atria and establishing a typical cardiac circulation pattern. If the septum primum does not close the opening, it results in a condition known as patent foramen ovale (PFO).