Which of the following divides the atrioventricular orifice on the fetal heart?

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Division of the atrioventricular orifice is achieved by the endocardial cushions. In early heart development, the common AV canal that connects primitive atrium to primitive ventricle is partitioned as the two endocardial cushions grow and fuse to form the atrioventricular septum. This fusion creates separate left and right AV orifices, giving rise to the mitral and tricuspid valve openings. The other structures have different roles: the septum primum drives atrial septation, the muscular septum partitions the ventricles, and the bulbus cordis contributes to the outflow tracts, not the division of the AV canal.