The primitive ventricle becomes divided by a septum, the septum inferius or ventricular septum, which grows upward from the lower part of the ventricle, its position being indicated on the surface of the heart by a furrow.
Its dorsal part increases more rapidly than its ventral portion, and fuses with the dorsal part of the septum intermedium.
For a time an interventricular foramen exists above its ventral portion, but this foramen is ultimately closed by the fusion of the aortic septum with the ventricular septum.