PLATES CXLII-CXLV) THE origin and nature of the phagocytes of the lung were an object of investigation by the earliest morbid histologists. Thus Slavjansky (1889) and von Ins (1878) indicated that these cells reached the alveolar spaces by diapedesis through the capillary walls, while Ruppert (1878) and Arnold (1882) traced their origin t o the lining cells of the alveolar walls. Since that time many investigations have been made on this subject ; reference should be made to the papers of Cappell (1929), Fried (1934) and Carleton (1934) for bibliographies.