Depending on the author, the alveolar phase begins at varying times. Probably in the last few weeks of the pregnancy, new sacculi and, from them, the first alveoli form. Thus, at birth, ca. 1/3 of the roughly 300 million alveoli should be fully developed. The alveoli, though, are only present in their beginning forms. Between them lies the parenchyma, composed of a double layer of capillaries, that forms the primary septa between the alveolar sacculi.
Already before birth these alveolar sacculi get to be increasingly complex structurally. Thereby, a large number of small protrusions form along the primary septa. Soon, these become larger and subdivide the sacculi into smaller subunits, the alveoli, which are delimited by secondary septa.
Ultrastructural investigations show that overall where such alveoli appear, they are surrounded by elastic fibers that form the interstitial septa between two capillary nets.
In the first 6 months, their number increases massively. This "alveolarization" and therewith the formation of secondary septa should - to a limited extent still - continue up to the first year and a half of life.