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N'J'S I k p a r l m r n l of A?intom?y, The University of Chicago mid Creiglrton J-ledicnl School TIIREE PLATES (SIX FIGURES)The questiori of the prcseiice or absence of a histological epithelium lining tho respiratory portion of the mainmalian lung cannot be settled until the changes wliich occur during the transition from late fetal to postnatal stages are thoroughly studied. Tt is not necessary to review here the controversial literature 011 the embryonic aiid postnatal mammalian lung as tliis has heen adoqnately treated in the woi-ks of Fried ( '34), Rremer ( '35) aiid Bargmann ( '36). The present study, based on close, successive stages of very fresh fetal and postnatal pig lungs, deals with the formation of the respiratory spaces and the fate of the connective tissue and the epithelium of the terininals of the bronchial trcc. Particular attention has been paid io the arrangement oi the blood capillaries in the developing respiratory portion of the lung. LLIATERTAT,S A S D I\J"THOT)SFetal pigs from sixty-one litters were secured at Cudahp's abbattoir, Omaha, so that tlic 130 fetal a i d three postnatal pigs prepared rcprcscnted thirty fetal stages based on millimeter crown-rump length, and two postnatal stages, as follows (number of litters in parentheses)