In bandicoots, implantation takes place 9.5 days after mating and birth occurs 3 days later. It is therefore of interest to examine the lungs of newborn marsupials to ascertain the stage of lung development with respect to other organs and other mammals in view of the short period of time allowed for maturation of lung tissue. The newborn bandicoot lung is composed of a primitive system of branching airways which terminate in large blind sacs; alveolar ducts or alveoli are not present. The terminating sacs, which are delineated by thick connective-tissue septae, have a highly vascularised internal lining. Thin squamous cells and numerous type-II "surfactant"-secreting cells constitute the air interface of the sac. Corticosteroids influence the development of the eutherian fetal lung by accelerating pulmonary maturation and increasing the synthesis of surfactant. Since recent evidence indicates that the adrenal of the bandicoot is active at, or before birth, it is possible that this organ also plays an important role in preparing the lungs of the marsupial for extra-uterine life in the pouch.