The pancreases of one to ten day old mice, Chinese hamters and Mongolian gerbils were examined by electron microscopy. Any essential species or sex differences were not recorded. In the ductules there were endocrine cells, sometimes reaching the lumen. Endocrine cells were also seen in buds on the ductules. Mitosis occurred in acinar, centroacinar, ductule, A—, B—, and capillary endothelial cells. The ductule cells exhibited cilia and signs of ceritriole replication. The various islet parenchymal cell types could be differentiated in all animals. Other findings were perinuclear fibril bundles, structural variation of the secretory granules, dilated intercellular spaces and signs of emiocytosis.

The results indicate that in the species investigated, new formation of endocrine cells occurs in the neonatal period both by proliferation and development of endocrine cells from the ductules and by mitotic division of the islet parenchymal cells. No morphologically detectable lesions were found in the islets of newborns of the Chinese hamster which has a hereditary trait for the development of diabetes mellitus.

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