Intracellular mucous inclusions. A feature of malignant cells in effusions in the serous cavities, particularly due to carcinoma of the breast.

Three cases of carcinoma of the breast are described, in which similar tumour cells were found in pleural or peritoneal fluid. The cells were characterized by the frequent presence of 'bull's-eye' vacuoles, in which a central spot was deeply stained by Giemsa, eosin or periodic acid-Schiff (PAS). In one of these it was shown by electron microscopy that the vacuoles are lined by microvilli, and that their mucinous content is condensed in the central area, presumably as a result of failure to discharge the secretion.