AbstractIn the monkey lung (Macacus nemestrina) canalization ofallbronchopulmonary segments begins simultaneously at the peripheral future‐alveolar‐duct portions of each segment and then continues progressively centralwards. Its simultaneous appearance in all segments of ahumanlung of the 17th postovulatory week (19th postmenstrual week) has now been confirmed (cf. Boyden, '74). The mode of canalization, as seen under the light microscope, is here demonstra