Representative images of female rat lungs at postnatal day 0 (P0), P6, P14 and P21. At P0, the rat lung is at the saccular stage of lung development. Alveolar formation takes place from approximately P4 to P14. At P21 alveoli have characteristic histological features of long, straight alveolar walls and numerous, long secondary septa. All panels original magnification 80 × (scale bar is 50 μm).