Pseudoglandular period 5-16 wk
Branching has continued to form terminal bronchioles. No respiratory bronchioles or alveoli present
Canalicular period 16-26 wk
Each terminal bronchiole divides into 2 or more respiratory bronchioles, which in turn divide into 3-6 alveolar ducts
Terminal sac period 26 wk
– birth Terminal sacs (primitive alveoli) form, and capillaries establish close contact
Alveolar period 8 mo
– childhood Mature alveoli have well-developed epithelial-endothelial (capillary) contacts