What is the name of the opening between the right and left atria present in fetal mammals?

The correct answer is foramen ovale, which is a crucial structure in fetal mammals. During development, the foramen ovale serves as a passageway that allows blood to flow directly from the right atrium to the left atrium, bypassing the lungs, which are non-functional in the fetus. This is essential because the fetus receives oxygenated blood from the placenta rather than from its own lungs.

After birth, the foramen ovale typically closes as the circulation changes and the infant begins to breathe air. The closure of this opening is important for establishing the normal separation between the oxygen-rich and oxygen-poor blood after birth.

The other choices refer to different anatomical structures or terms that do not relate to the fetal circulatory system. For instance, the foramen rotundum is a small opening in the sphenoid bone and pertains to cranial anatomy, while the foramen magnum is the large opening at the base of the skull where the spinal cord passes. The atrio-ventricular septum is a part of the heart that separates the atria from the ventricles but does not act as a direct passageway between the atria.