Final answer:
The foramen ovale and the ductus arteriosus are the two structures in the developing fetus that allow most of the blood to bypass the lungs.
Step-by-step explanation:
The two structures in the developing fetus that allow most of the blood to bypass the lungs are the foramen ovale and the ductus arteriosus. The foramen ovale is a shunt that connects the right atrium to the left atrium, allowing blood to flow directly from the right side of the heart to the left side, avoiding the pulmonary circuit. The ductus arteriosus diverts some of the blood from the pulmonary trunk to the aorta, again bypassing the lungs.