A 43-year-old woman of Scandinavian descent complains of constant tiredness, light-headedness, and occasional palpitations and shortness of breath while ascending the stairs. Physical examination shows pallor of the oral mucosa and a raspberry-red tongue (glossitis). Neurologic examination reveals paresthesias, numbness, decreased vibration sensation, and loss of deep tendon reflexes. The results of laboratory studies include hemoglobin of 7.2 g/dL, WBC of 4,500/mL, platelets of 140,000/mL, erythrocyte folate of 220 ng/mL, serum vitamin B12 of 40 pg/mL (normal >200 pg/mL), serum anti-intrinsic factor of 1 :128, and serum anti-parietal cell antibody of 1 :64. Examination of peripheral blood shows macrocytic anemia, with poikilocytosis of RBCs and hyper segmented neutrophils. Atrophic gastritis is diagnosed by gastric biopsy. Bone marrow examination in this patient will reveal which of the following pathologic findings?