submucosa is less involved, as though the necrotizing agent had not penetrated to the same depth and the serous reaction beneath were actually a beneficent exudate. These blisters are in contrast with the deeper areas where the fibrinous mass, mixed with the dead tissue, forms an intensely staining ring or band, which extends through the bronchiolar wall even to the surrounding alveoli. In the early stage of this process one of the most outstanding features is the absence of polynuclear leucocytes in the reactionary process, but gradually as the dead tissue sloughs away, these cells wander into the exudate and form a purulent ring, more intense in the lumen, but extending for a variable distance through the still viable wall of the structure (47) (Fig. IX). Later mononuclear cells accumulate in this wall and occur either as a diffuse mottling or as circumscribed foci in the muscle and submucous layer of the bronchiole, just as they do in the trachea. Occasionally, a striking change is found in a small bronchiole within a portion of the lung which is otherwise uninvolved by an inflammatory process. Perhaps the alveoli were the seat of a change which has subsided, but, whatever the history, the purulent mass in the bronchiole and involving its wall, stands out effectively (Fig. X). Sooner or later, with the subsidence of the irritating agent, repair begins in the bronchus or bronchiole. If its walls have been destroyed and the lesion has extended into the surrounding alveoli to form an abscess of greater or lesser extent, or if the necrotizing process has been superficial and confined to the epithelium in large part, the reparative process is very much the same. Mitotic figures in the fibroblasts and in the endothelial cells of the capillaries abound in the young granulations (Fig. XI). However, the granulation tissue does not have an unrestricted path of growth, for if a remnant of epithelium remains, this is stimulated to grow probably in this disease as in no other. Mitotic figures are common and the young epithelial cells stretch across the denuded submucosa or granulation (Fig. XLVIII) and extend downward into the surrounding alveoli, not only as strands, but also as solid nests of cells (47) (Fig. XLIX). The bronchioles, therefore, show changes dependent upon the extent of the damage suffered by their walls. The vast majority, in all probability, will be restored; but if the wall has actually been necrotized, the bronchioles may be converted into small, saccular, bronchiectatic cavities (48, 110, 162) (Figs. XII and L), or obliterating bronchiolitis may result from the organization of the exudate within their lumina (82) (Fig. XI). The importance of the epithelial proliferation cannot be ignored; in many cases, it invades the surrounding lung tissue and a typical, histological picture results—an infiltrating, malignant, epithelial neoplasm (Figs. XLVIII and XLIX). _Summary._