The kinds of cells and the sequence in which they appeared in the inflammatory exudate were studied in a series of experimentally produced pneumonic lesions in dogs. There was a gradual and progressive change in the character of the exudate and the kinds of cells as the disease progressed. The microscopic findings could be more closely correlated with the age of the lesions than with their gross appearance or with the clinical condition of the animal at the time of death. The cells in the exudate came principally from the blood. The polymorphonuclear leucocytes were gradually replaced by larger phagocytic mononuclear cells. These were derived chiefly from the hypertrophy and transformation into larger phagocytic cells of the lymphocytes and monocytes of the blood after they entered the air spaces along with the polymorphonuclear leucocytes in the early stages of the disease. To follow the development of the hematogenous exudate cells into macrophages in the dog, the pneumonic process must be studied from its earliest inception and at close stages during the first 36 hours of the disease. The local septal cells contributed only in a minor way to the origin of the macrophages. Their principal reaction appeared to be one of enlargement without detachment from the alveolar walls. Consolidation of the lungs occurred as a result of the spread of the pneumococci through the air spaces by direct passage of the infected edema fluid from alveolus to alveolus through the pores of Kohn and from bronchiole to bronchiole from aspiration during breathing and coughing. The similarity of the histogenesis of the exudate cells in this series of experimentally induced pneumonic lesions in the dog and in those which occur spontaneously in man was discussed.

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