Lung development in marsupials is generally slow. During the
early postnatal lung development, the saccules become subdivided
by septal crests and decrease in size. The first true alveoli, identified
by the presence of single-capillary septa, are present at 28 days in
the opossum and at 65 days in the tammar wallaby. A typical alveolized
lung structure, characterized by the presence of respiratory
bronchioles, alveolar ducts and alveolar sacs can be seen at 56 days
in the opossum and at 142 days in the tammar wallaby. The postnatal
lung development in altricial eutherians is much faster than
in marsupial species. The first signs of the transformation from
the saccular to the alveolar stage can be observed at 2 days after
birth in the golden hamster and at the end of the first week a typical
alveolized lung structure is present. Intermediate species, like
the tree shrew, reach an alveolized lung structure within 4 days
after birth and precocial eutherians are already born at this stage
of development (Fig. 4).