Azygos lobe

In human anatomy, an azygos lobe is a normal anatomical variation of the upper lobe of the right lung. It is seen in 1% of the population. Embryologically, it arises from an anomalous lateral course of the azygos vein, in a pleural septum within the apical segment of the right upper lobe or in other words an azygos lobe is formed when the right posterior cardinal vein, one of the precursors of the azygos vein, fails to migrate over the apex of the lung and penetrates it instead, carrying along two pleural layers that invaginates into the upper portion of the right upper lobe. As it has no bronchi, veins and arteries of its own or corresponding alteration in the segmental architecture of the lung, so it is not a true (misnomer), or even accessory, pulmonary lobe, but rather an anatomically separated part of the upper lobe.
Clinical significance.
An azygos lobe is usually an incidental finding on chest x-ray or CT scan, and is not associated with any morbidity. However, it can cause technical problems in thoracoscopic procedures.