A later phase in this series of changes is represented by new formation of fibrous tissue. The bronchial walls and interalveolar septa are thickened by proliferating fibrous tissue, young fibroblasts and newly formed collagen fibrils being abundant (Fig. 28; also Fig. 30). This increase of fibrous tissue is especially noteworthy immediately surrounding the walls of the small bronchi, which are often considerably dilated, and about the smaller of those bronchi which have cartilage; with thickening of alveolar walls immediately adjacent to the bronchus every stage in the obliteration of the alveoli may be found. Their walls are thickened and their lumina are diminished in size and often flattened in a direction concentric with the bronchus. Such atrophied alveoli lined by cubical epithelial cells occurring within the thickened peribronchial fibrous tissue give evidence that this tissue has replaced alveoli. Alveoli surrounding and within the new fibrous tissue are frequently filled with fibrin, and organization indicated by penetration of fibroblasts and capillaries into the fibrin may be far advanced. There is some increase of perivascular and interlobular tissue. The bronchiectasis which is almost invariably found with unresolved bronchopneumonia has been described. Squamous transformation of epithelium (page 251) is frequently found in association with the chronic bronchitis of unresolved pneumonia. =Organizing Bronchitis and Bronchiolitis.=—When the bronchial epithelium is destroyed, fibrin is deposited upon the denuded surface and may partly or completely fill the lumen of the bronchial tube. The plug of fibrin is adherent to the underlying tissue wherever epithelium is lost but is separated from the bronchial wall by a well-defined space where epithelial lining is still intact. Fibroblasts promptly migrate from the wall of the bronchiole into this fibrin, and fibroblasts, fixed during ameboid movement, are irregularly elongated in a direction toward the fibrin.