The reinforcement of the fissured bronchial wall by new formation of fibrous tissue, by thickening of the interalveolar walls and by organization of fibrin within the alveoli is well shown after four weeks (Autopsy 425; Fig. 28). There are spherical bronchiectatic cavities more than a centimeter in diameter surrounded by a dense fibrous wall in which are atrophied alveoli lined by epithelium of cubical form. Occasionally, the fibrous wall is interrupted and alveoli, plugged with organizing fibrin, are in immediate contact with the lumen. When these plugs of fibrin which are slowly absorbed disappear, evidence of preexisting rents in the bronchial wall are lost, and there are in this lung bronchiectatic cavities of which the wall is a continuous circle of dense fibrous tissue. [Illustration: Fig. 21.—Bronchiectasis with fissures extending through the bronchial wall into alveolar tissue which is the site of fibrinous pneumonia; epithelium has grown down into these fissures and has covered the exposed surfaces. Autopsy 463. ] [Illustration: Fig. 22.—Regeneration of epithelium over fissures which have been formed in the wall of a bronchus; the epithelium in the neighborhood of and within the fissure is squamous. ] Epithelium lining the dilated bronchi is at times completely destroyed (Fig. 28), but more frequently it persists in part. That which remains has almost constantly the character of squamous epithelium (Figs. 22 and 23). The lowermost cells are cubical; those above them are polygonal, tending to become flatter as the surface is approached; upon the surface are cells often much flattened and occasionally they have lost their nuclei and stain deeply with eosin as the result of superficial necrosis. The change should not be regarded as metaplasia, for the epithelium assumes this squamous type when the superficial columnar cells have been lost. Actual necrosis of superficial ciliated columnar cells is occasionally seen (Autopsy 352); injured cells have separated from one another and desquamated into the lumen of the bronchus. The epithelium which remains after the superficial cells are lost consists of cells which become flatter from base to surface, but the intercellular bridges characteristic of the epithelium of the skin are not found. When epithelium is in process of regeneration, a layer gradually diminishing in thickness extends over the denuded surface, the advancing edge being formed by very flat cells in a single layer. The epithelium growing into fissures which have penetrated the bronchial wall may completely cover the exposed alveolar tissue. The newly formed epithelium may follow a fissure into an alveolus which has been opened and come into contact with the fibrin which fills the alveolus. [Illustration: Fig. 23.—Squamous epithelium growing over the defect in the bronchial wall shown in Fig. 22 more highly magnified; squamous epithelium is present above and columnar epithelium below. ]