The lung is qualitatively different at birth, compared to adult life, in many species. In the neonatal period, the peripheral part of the acinus is formed by primary saccules, and it is from the walls of these that alveoli develop postnatally in rats, mice, and rabbits. Alveolar development starts in utero in man and probably in several other species, including sheep, cat, and, probably, mammals with relatively advanced physical development at birth. A sequence of events is well established in rats and mice in the postnatal period. A phase of expansion first occurs, during which lung growth lags behind the increase in body weight and lungs enlarge primarily by expansion, but new tissue is laid down in the saccular wall. This is succeeded by a phase of tissue proliferation during which the saccule is subdivided by numerous secondary crests that develop in the saccular wall and result in the formation of alveoli. During this stage, the rate of lung growth exceeds the rate of body growth and there is a rapid increase in alveolar surface area. Elastic tissue plays a key role in the development of alveoli, and elastic fibers lie in the free margins of the secondary crests and around the mouths of the alveoli. At approximately 2 weeks of age, the rate of cellular multiplication and formation of alveoli is markedly diminished and the lung grows mainly by cellular enlargement, dilatation, and modification of pre-existing structures...