As the right atrium grows in size by incorporating the right sinus horn, the left atrium also enlarges by absorbing the primitive pulmonary vein into its wall.
During the 4th week a pulmonary vein develops as an outgrowth of the posterior atrial wall, so that originally only one pulmonary vein opens into the posterior wall of the developing left atrium. The primitive pulmonary vein promptly divides into right and left branches, which bifurcate again to produce a total of four pulmonary veins. These veins grow toward the lungs and become connected to the veins developning
in the mesoderm surrounding the bronchial buds.
Due to rapid expansion of the left atrium during the 5th week, the pulmonary vein and its' branches are gradually absorbed into the wall of the left atrium. As mentioned above, initially pulmonary venous system opens into the left atrium through a single orifice, then transiently through two orifices, and finally through four orifices of the definitive pulmonary veins.
Most of the wall of the definitive left atrium is smooth because it is derived form the pulmonary veins and its branches. The trabeculated part of the primitive left atrium is displaced ventrally and to the left, where it becomes the left auricle.