Dense, homogeneous granules of 250 to 350 mµ in diameter have been found inside the cavities of the endoplasmic reticulum in the acinar cells of the pancreas of the guinea pig. They apparently correspond to the fine granules described in the same material in light microscopy by Mankowski and Bensley. The location of these granules indicates that the endoplasmic reticulum is a cavitary system and suggests its participation in secretory processes.

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