1. Results from the electron microscopic observations of the autonomic nerve terminals in general were summarized and discussed with relation to the concept of “the autonomic ground plexus” proposed by HILLARP (1959, 1960).
2. The fine structure of adrenergic and cholinergic nerve terminals and their distribution within the vertebrate heart have been described. From the amphibian, avian and mammalian myocardium, evidence has been obtained which indicates the two types of autonomic axon terminals to exist in pairs sharing a common Schwann cell sheath.
3. Special attention was focused upon the particularly dense innervation of the cardiac pacemaker regions which have been observed in the heart of lower as well as higher vertebrates.
4. Very little information has been available concerning the fine structure of sensory nerve endings within autonomic effectors. Axon profiles, packed with an unusual abundance of mitochondria and a few vesicles and tentatively interpreted as representing a peripheral portion of the cardiac afferents, have been found in the guinea pig pulmonary valve and in the human ventricular myocardium.

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In biopsy specimens of pancreas taken from five patients with gastric ulcer the pancreatic islets were electron microscopically examined.
1. The human islet cells are classified into B, A and D cells chiefly on account of the ultrastructural differences of their specific granules. In β-granules (300-370mμ) one or several crystalloid cores of moderate electron density are loosely encased in a membrane sac, though atypical β-granules containing one spherical core are not uncommon. The α-granules (450-650mμ) contain a single spherical core of high electron density which is in larger granules loosely, but in smaller ones closely, encompassed by a membrane sac. The space between the core and the sac appears empty after single fixation in osmic acid but moderately electron opaque after double fixation with glutaraldehyde and osmic acid. The δ-granules (450-650mμ) are, in the osmic acid fixed preparations, composed of a spherical core of low electron density and an interrupted membrane sac applied very closely to the core; after doublefixation the core appears more electron dense and a continuous sac is attached directly to it.
2. The release of specific granules by emiocytosis has not been supported by the present observations, but figures suggesting a diacrine secretion through intracytoplasmic dissolving of the granules, e.g. density decrease and disintergration of the core and perforation or disappearance of its membrane sac, have been common to all islet cell types. Especially in the D cells the density of the δ-granules are conspicuously lowered as they become matured and their membranous sacs are often perforated or lost in the most electron lucent ones.
3. The Golgi complex which is largest in B cells is located either on the vascular side of the cell with an accumulation of the specific granules or on the opposite side. The formation of the specific granules within the Golgi complex seems to be probable in human three islet cell types; minute granules appear in dilated portions of the Golgi cisternae or, contained in a smooth membraneous sac, in close proximity to the Golgi complex. A single centriole is found occasionally within the Golgi area.
4. All three islet cell types are rich in the elements of granular endoplasmic reticulum and free ribosomes aggregated usually into small clusters.
5. Mitochondria are variable in shape and size especially in the B cells; round and irregular mitochondria with obscure cristae may occur.
6. The human islet cells are characterized by occurrence of lipoidal bodies. In the B cells they (2-3μ in diameter) occur most abundantly and are bounded by a double membrane similar to mitochondrial membrane; their matrix of moderate density contains vesicles or vacuoles usually in a considerable number which do not bulge into the surrounding cytoplasm. In A and D cells the lipoidal bodies occur less frequently; their matrix is highly electron dense and contains relatively few vacuoles of larger size and vesicles which bulge extensively into the surrounding cytoplasm. The lipoidal bodies of the B cells seem to originate from the swollen mitochondria whereas those in A and D cells from the Golgi complex. The fate and significance of the lipoidal bodies of the human islet cells have been discussed.
7. It has been emphasized that the human A and D cells are similar in the ultrastructural properties of their secretory granules and the lipoidal bodies. In the human endocrine pancreas both islet cell types seem to be akin to each other.

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Structural details of the surface of the developing deciduous tooth enamel were studied under the scanning electron microscope using materials from human fetuses ranging in age from 6 to 8 months. The excised dental germs were fixed either in 2.5% glutaraldehyde or 10% neutral formalin solution. In a number of specimens fixed, treatment by 1:2 diamino-ethane was conducted to remove organic materials from the surface of enamel.
1. The surface of developing enamel appeared to consist of a regular arrangement of depressions, each of which was concave due to the formation of the interrod portion which always preceded that of th rod ends. The size and depth of these depressions varied according to the stage of enamel development.
2. In immature stages of enamel development the depressions were always deeper than those seen in the mature enamel and showed either tapared cylindrical or wedgelike forms. The floor of the depressions were rough-surfaced, being covered by particles smaller than 100mμ in diameter. On the walls of the depressions there were numerous structures of varying size acicular to columnar in form. Many depressions also contained other structures, such as spherical granules and amorphous lumps. In the specimens treated with organic solvent these structures had almost disappeared. Instead, there were crystalline structures of enamel matrix, most of which had the appearance of well defined flakes in relief.
3. It was demonstrated by an electron microanalyser that the Ca concentration in the rod ends in immature stages of enamel development was much higher than that in interrod portions.
4. In the boundary zone between immature and mature enamel the depressions became considerably shallower and showed a more flattened floor.
5. The surface of mature enamel lacked even the shallowest kind of depressions as observed in the boundary zone, and there was seen only undulation of very slight concavities and elevations arranged alternately.

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The brains of the adult lampreys, Lampetra japonica, were examined from a phylogenetic point of view with the aid of fluorescence histochemical method for biogenic monoamines (FALCK and HILLARP), and four groups of fluorescent cells were revealed.
1. Neurons beneath the layer of the olfactory epithelium were found to fluoresce. The axons of these cells entered the brain through the olfactory nerve and presumably made synapses with other neurons.
2. In the optic lobe pedunculus and medulla oblongata fluorescent neurons were arranged along the wall of the ventricle, whose axons were impossible to trace precisely.
3. Fluorescent cells with a peculiar process were found in the pineal body and assumed to be of sensory nature because of their characteristic arrangement and form.
4. Fluorescent cells of presumable bipolar form were distinguished from the ependymal cells. These cells were believed to be monoamine containing cells by the result of fluorescence specificity test. The processes of these cells protruded into the lumen of the ventricle with or without droplet or tuft-like structure at the end of the processes. A release of the substance by these processes into the the ventricle was postulated.
5. A homology of the periventricular fluorescent cells of the lamprey to the catecholamine containing cells of the teleost hypothalamus was discussed.
In the present investigation, fluorescent cells or fibers were recognized neither in the hypophysis nor in the preoptic nucleus.

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