Five stages of lung development. Stages of lung development in humans: Diagrammatic timeline of the developmental organisation of the mammalian respiratory system. At the embryonic stage, the major airways are formed. During the canalicular stage epithelial differentiation occurs and the air-blood barrier is formed. In the saccular stage of lung development, air spaces expand and finally at the alveolar stage, secondary septation occurs. Reprinted from  with permission from Elsevier, copyright (2007).