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At around E16.5 in the mouse, lung development switches from branching morphogenesis to the canalicular and saccular stages. These, in turn, lead to the final process of alveologenesis (also known as alveolization) that generates the functional units in the lungs, for gas exchange. In mice, this final stage occurs postnatally (P5–30), but in humans some alveoli form before birth and the process continues for many months afterwards.