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 0.3% 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 as the azygous fissure, that invaginates into the upper portion of the right upper lobe. Clinical significance 
An azygos lobe is usually an incidental finding on chest x-ray or CT scan. It is asymptomatic and not associated with any morbidity. However, it can cause technical problems in thoracoscopic procedures. The presence of the azygos lobe could alter the normal location of the superior vena cava or may be associated with other anomalies, including esophageal atresia or intrapulmonary right brachiocephalic veins. Additional images

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Further reading 

 
 

Congenital disorders of respiratory system
Veins of the torso