A 61-year-old man gives a history of back pain for several months. A plain film radiograph of the spine reveals several 1 to 2 cm lytic lesions of the veebral bodies. He has no lymphadenopathy or splenomegaly on physical examination. Serum chemistries show sodium 140 mmol/L, potassium 4.4 mmol/L, chloride 101 mmol/L, CO2 26 mmol/L, glucose 78 mg/dL, urea nitrogen 49 mg/dL, creatinine 5 mg/dL, total protein 8.3 g/dL, albumin 3.7 g/dL, alkaline phosphatase 176 U/L, AST 45 U/L, ALT 22 U/L, and total bilirubin 1.2 mg/dL. He recently developed a cough productive of yellowish sputum, and a sputum culture grew Streptococcus pneumoniae. During the course of his hospitalization, a bone marrow biopsy is performed. The biopsy is most likely to show: