The epithelial proliferations, which are so typical of jagziekte, are chiefly of alveolar origin. They appear in areas of infiltration and exudation identical histologically with those occurring in sheep which are to all appearances normal but which are derived from flocks in which the disease is endemic. They become very extensive and quite atypical in structure, but they never give rise to metastases, even to the adjacent lymph glands. The primary infiltration of the interalveolar tissue continues and may lead to fibrosis and extensive consolidation, or leucocytes may accumulate in large numbers, penetrate into the alveoli, and form areas of acute pneumonia. In fairly advanced cases of jagziekte it is common to find many stages of the reaction in one and the same lung. While it is possible that the disease may be due to a single and specific virus, and the epithelial lesions are certainly of a very special character, the pathological complex as a whole is such as might be caused by a variety of predisposing and exciting factors operating over a considerable period of time.