Thinning of epithelium, terminal sac formation.

Acinar formation and capillarisation, type I and II alveolar epithelial cells begin to differentiate

Definitive alveoli begin to appear, secondary septation occurs along with expansion of airspaces

The lungs begin to bud from the foregut endoderm, with formation of the upper respiratory tract

Airway division completed, 25, 000 terminal bronchioles, cartilage and smooth muscle derived from mesenchyme