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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.

PLATES 64 TO 66The cytological problems presented by the synapse were carefully considered by Bodian (6) in his review of 1942. He indicated that it is desirable to know more about "the ultrastructure of the membranes concerned, the quantitative relations existing between surfaces of axon terminals and the surfaces Of cells of termination, the intimate relationships of myelin and of glia to synaptic surfaces, thetypes, approximate numbers, and specific sites of origin of endings on cells of each type, the electrical properties of cellular elements and intracellular materials, and the localized concentrations and site of action of both drugs and physiological substances which may produce or modify neuronal excitation, under varying conditions." Thanks to recent advances in neurophysiology (11), neuropharmacology (12,22), and neuroanatomy, it is now possible to give answers to some of these problems, although the answers are by no means complete. The following paper reports an electron microscopic study of synaptic terminals in the medulla oblongata, cerebellar cortex, and cerebral cortex of the rat. The information presented is not sufficient for generalization to all types of synapses, but a remarkable uniformity in architecture and internal structure has been found among the morphological types of junctions studied. It remains to be seen whether this uniformity can be extended to synapses in all parts of the nervous system and to synapses with different pharmacological and physiological properties.In addition to the preliminary report on similar synapses in abstract form (20), a number of electron microscopic studies of other synapses has appeared in the past few years. Robertson has studied axo-axonic synapses in the stellate ganglion of squid and in the abdominal ganglia of crayfish (27) as well as neuromuscular junctions in the chameleon (28). SjSstrand (31) has examined the complex junctions between the rod cells and bipolar cells in the retina of

Axon hillocks and initial segments have been recognized and studied in electron micrographs of a wide variety of neurons. In all multipolar neurons the fine structure of the initial segment has the same pattern, whether or not the axon is ensheathed in myelin. The internal structure of the initial segment is characterized by three special features: (a) a dense layer of finely granular material undercoating the plasma membrane, (b) scattered clusters of ribosomes, and (c) fascicles of microtubules. A similar undercoating occurs beneath the plasma membrane of myelinated axons at nodes of Ranvier. The ribosomes are not organized into Nissl bodies and are too sparsely distributed to produce basophilia. They vanish at the end of the initial segment. Fascicles of microtubules occur only in the axon hillock and initial segment and nowhere else in the neuron. Therefore, they are the principal identifying mark. Some speculations are presented on the relation between these special structural features and the special function of the initial segment.