Final answer:
The primitive ventricle is the first structure in the partitioning of the primordial ventricle, forming the left ventricle in the adult heart. The foramen ovale, located above, allows fetal blood to bypass the lungs and closes after birth.
Step-by-step explanation:
The first structure in the partitioning of the primordial ventricle is the primitive ventricle, which eventually forms the left ventricle of the fully developed heart. The hole above this structure in the fetal heart is the foramen ovale, which allows blood to pass directly from the right atrium to the left atrium, bypassing the pulmonary circuit. After birth, this opening closes and becomes the fossa ovalis.