
The Formation of the Ventricles and Major Arteries
Starting from the 4th week, a septum begins to form within the primitive ventricle, giving rise to the interventricular septum. This structure is primarily muscular, but its upper section is completed by a thin fibrous portion known as the membranous septum.
This fibrous portion originates from the partitioning of the arterial bulb, which also gives rise to the pulmonary artery and the aorta, two of the main vessels in the circulatory system.