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The pancreas of human fetuses aged from 6 to 38 weeks were observed for the developmental study ,of the primitive pancreatic ducts, acini, islets of Langerhans, the time of appearance, and morphology of the islet cells. 1. The fusion of the ventral and dorsal pancreata has been encountered on the sixth week of fetal life as an organ containing the primitive pancreatic ducts surrounded by a large amount of loose con nective tissue. The epithelial cells of the ducts are abundant in glycogen. They fused with the neighboring cells with junctional complexes at free surface and with desmosoms at some place deeper into the intercellular space, and the other portion of the intercellular space is slightly widened. The ducts branch to form the lobules and then the lobes in accordance with the increase of fetal age. 2. In the pancreas of the twelfth week of fetus the primitive acinar cells, having many oval or fusiform granules and lamellated granular endoplasmic reticulum, appear around the terminal portion of the primitive pancratic duct and show rapid growth thereafter. 3. The endocrine cells appear in the order of alpha, delta, and beta cells, on the eighth and half, twelfth, and thirteenth week of fetuses, respectively. They proliferate to form the islets which have alpha and delta cells as the major types of the islets, and then become gradually degenerate. New islets which look like the adult islets, and have beta cells as the major type, begin to appear. The alpha cells have small, round granules and the beta cells have granules with crystalline structure as a core.
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