In mammals there is a high variability in the degree of lung maturation
at birth, reflecting the general development of the neonates
(Engel, 1962). In the most immature neonates of monotremes and
marsupials we find the earliest stage of lung development in which
mammals are born: the lung structure is at the early saccular
phase (Fig. 3A–C). The lung is composed of a primitive system of
short branching airways which terminate in large terminal saccules
providing only a small surface area, but they are sufficient
for gas exchange. In monotremes, represented by the platypus
(Ornithorhynchus anatinus), the saccules of the lung have a diameter
of 320 ± 13 m (Ferner et al., 2009a; Fig. 3A).