The ergastoplasm (granular endoplasmic reticulum) of parotid acinous cells of the mouse is described with special emphasis on unusual forms of this membranous system. In the majority of the acinous cells the ergastoplasm appeared in sections to consist of a large number of separate flattened cisternae. In some acinous cells, however, the ergastoplasm was disposed as a very small number of large membranous formations. Although extensive and complicated in form, these latter formations could be seen, from the examination of a single section, to consist of a single expanse of membrane continuous with the nuclear envelope. Certain acinous cells exhibited ergastoplasmic formations which appeared to represent intermediate stages of a fragmentation or metamorphosis of the larger formations toward the more usual form of ergastoplasm. These observations suggest the possibility that the early elaboration of ergastoplasm consists in the production, in relation to the nuclear envelope, of large, complicated membranous formations that subsequently sever their connection with the nuclear envelope and form a large number of separate, or tenuously connected, cisternae. The majority of the large, complicated ergastoplasmic formations were seen in parotid glands of mice that had been starved for 4 days and subsequently fed for a variable number of hours, but some were found in glands that were not subjected to experimental treatment. The tissues studied were prepared for electron microscopic examination by fixation in osmium tetroxide, dehydration in alcohol, imbedding in butyl methacrylate, sectioning with a glass knife, staining with lead hydroxide, and sandwiching with formvar.

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