1. Thin sections of representative neurons from intramural, sympathetic and dorsal root ganglia, medulla oblongata, and cerebellar cortex were studied with the aid of the electron microscope.
2. The Nissl substance of these neurons consists of masses of endoplasmic reticulum showing various degrees of orientation; upon and between the cisternae, tubules, and vesicles of the reticulum lie clusters of punctate granules, 10 to 30 mµ in diameter.
3. A second system of membranes can be distinguished from the endoplasmic reticulum of the Nissl bodies by shallower and more tightly packed cisternae and by absence of granules. Intermediate forms between the two membranous systems have been found.
4. The cytoplasm between Nissl bodies contains numerous mitochondria, rounded lipid inclusions, and fine filaments.