In which stage of lung maturation is the blood–air barrier established?

D. The simple cuboidal epithelium within the terminal sacs differentiates into pneumocytes within the terminal sac period. The rapidly proliferating capillary network makes intimate contact with the terminal sacs, and the blood–air barrier is established with type I pneumo-cytes. These events take place in the terminal sac period, which runs from embryonic week 24 until birth.