Comparison of two functioning human pancreatic islet cell tumors with adjacent uninvolved islets enabled identification of various islet cell types on the basis of their fine structure. The evidence suggests that mature secretory neoplastic beta cells arise by differentiation from immature budding ducts in a germinal region of the tumors, through a process resembling that of islet differentiation in embryos. This work was undertaken because biopsy tissue presented an opportunity to study the

more »

... e structures of different cells in human pancreatic islets and to compare these with the structures of tumor cells. The tissue also permitted study of the secretory characteristics in functioning endocrine normal and neoplastic cells. CASE HISTORIES Case I A 66-year-old white man underwent partial pancreatectomy for the removal of a tumor believed responsible for episodes of hypoglycemia. During the 4 hours before removal of the lesion, he received approximately 40 gm. of dextrose intravenously. Before dextrose administration, glycemia was 75 mg. per cent; whereas s and 9 hours after removal of the tumor, it was II2 and i65 mg. per cent, respectively. Measure-