From the last trimester whole clusters of sacs form on the terminal bronchioli, which represent the last subdivision of the passages that supply air. In the saccular phase the last generation of air spaces in the respiratory part of the bronchial tree is born.
At the end of each respiratory tract passage smooth-walled sacculi form, coated with type I and type II pneumocytes. The septa (primary septa) between the sacculi are still thick and contain two networks of capillaries that come from the neighboring sacculi. The interstitial space is rich with cells and the proportion of collagen and elastic fibers is still small. This matrix, though, plays an important role for the growth and differentiation of the epithelium that lies above it.
At the end of this phase the interstitial fibroblasts begin with the production of extracellular material in the interductal and intersaccular space.
At birth, i.e., at the end of the saccular phase, all generations of the conducting and respiratory branches have been generated. The sacculi are thin, smooth-walled sacks and correspond to the later alveolar sacculi.