In order to obtain insight into the pathogenesis of these lesions, it is desirable to imitate the historical development of knowledge concerning the characters and causes of disease, namely, first to define accurately the lesions concerned and later to determine with what microorganisms these lesions are associated. The difficulties of this undertaking are increased by the multiplicity of the microorganisms concerned and by the well-known truth that the same microorganism, _e. g._, the tubercle bacillus, may produce widely different anatomic lesions. In the table of autopsies the following lesions are listed: =Column 7. Purulent bronchitis.=—“P” indicates that the small bronchi contain mucopurulent fluid. =Column 8. Lobar pneumonia.=—The occurrence of the lesion is indicated by the plus sign (+). =Column 9. Peribronchiolar consolidation.=—The presence of nodular patches of consolidation about respiratory bronchioles is indicated by the plus sign (+) when the lesion has been recognized at the time of autopsy. When the lesion has been first recognized by microscopic examination the letter “M” is used. =Column 10. Hemorrhagic peribronchiolar consolidation.=—The occurrence of this lesion which represents the preceding on a background of hemorrhage is indicated by the plus sign (+). =Column 11. Lobular consolidation.=—The presence of the lesion is indicated by the plus sign (+). =Column 12. Peribronchial consolidation.=—Peribronchial pneumonia recognized at the time of autopsy is indicated by the plus sign (+). Peribronchial pneumonia recognized microscopically is indicated by “M.” The presence of peribronchial hemorrhage without consolidation is indicated by “h.” =Column 13. Abscess formation with pneumonia.=—Suppuration with abscess formation almost invariably just below the pleura is indicated by the plus sign (+). Necrosis of lung tissue recognized microscopically and unaccompanied by suppuration is indicated by “N.” =Column 14. Suppurative interstitial pneumonia.=—This lesion invariably associated with suppurative lymphangitis is indicated by the plus sign (+). =Column 15. Multiple abscess in clusters.=—Abscesses in clusters about a bronchus of medium size are indicated by the plus sign (+). =Column 16. Empyema.=—The presence of the lesion is indicated by “E.” =Column 17. Bronchiectasis.=—“B” indicates the lesion. =Column 18. Unresolved bronchopneumonia.=—Presence of the lesion is indicated by the plus sign (+). =Column 19. Organizing bronchitis and bronchiolitis.=—“O” indicates the lesion. The lesions of columns 7 to 12 are acute inflammatory processes, columns 9 to 12 represent different types of bronchopneumonia. Columns 13 to 15 represent suppurative lesions. Columns 17 to 19 represent chronic lesions. A survey of the table shows the predominance of acute lesions in the early period of the study and the gradual increase of chronic lesions.