Changes occurring in the rat pancreatic acinar cell during mitosis have been studied by electron microscopy. Special attention has been given to those occurring in the Golgi complex.There is an initial increase in the number of small vesicles between the rough-surfaced endoplasmic reticulum and the saccules of the Golgi apparatus. These persist throughout the mitotic cycle. The Golgi apparatus becomes considerably smaller in metaphase. In anaphase the normal stacked appearance of its saccules is lost. In telophase the Golgi complex is restored to its normal interphase appearance and the small vesicles may contribute both to the reformation of its saccules and to the cleft forming between the daughter cells. Condensing vacuoles and zymogen granules appear to form throughout mitosis, although they have a much greater heterogeneity of size than in the interphase cell. The possible significance of some of these changes is discussed.

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