The pattern of the terminal air spaces in a premature infant of 30–32 weeks that lived nineteen and a quarter hours

The availability of a perfectly preserved lung from a premature infant of 30–32 weeks that lived nineteen and a quarter hours has made possible the first wax reconstructions of the terminal air spaces in the canalicular period of fetal growth. Here, as in the newborn, the most distal respiratory bronchioles give rise to two pairs of clusters of saccules, but with this difference: each saccule (lined by flat epithelium) is separated from others by a spongy, vascular, connective tissue matrix. By reconstructing just the cavities of the saccules one can study the bizarre shapes presented by the saccules. In pattern the latter closely resemble peripheral alveolar saccules of the child lung modelled by the writer. Seemingly, therefore, this fetal period is the one in which the terminal generations of branches are being elongated and their shapes determined by invading capillaries and connective tissues. Clinically, one may conclude that peripheral diffusion of gases (in the infant or adult lung) does not follow any such simple dichotomous pattern of flow as has been hitherto assumed.