Photomicrographs of peritoneal surfaces in mice carrying Lucite 21× 2 mm plastic discs. (A) 3616 1.5 × 2× Day 315. Section through mesenteric fat and intestine showing patchy fibrous deposition on mesothelial surfaces of mesenteric fat. Note that much of the surface is not covered. (B) 3173 20 × 10× Day 214. Region near the mesenteric attachment site containing three polyp-like structures. The two light staining polyps contain fibrinoid tissue covered with mesothelium. The third polyp is invaded by plasmacytoma cells. (C) 3164 20 × 5× Day 214. A patch of fibrous tissue on a fat surface on the left. The tissue contains numerous lymphocytes. (D) 3333 20 × 5× Day 258 Vascularized polyp-like structure arising from the omentum that contains a focus of atypical plasma cells. This was the only plasma cell lesion found in this mouse.