Recent progress in endocrine physiology showed six different hormones secreted from the anterior pituitary gland, but their cellular sources were not completely elucidated, when the morphological technique was confined to the light microscopy. Application of the electron microscope is very advantageous to the field of functional morphology of the pituitary gland. Within a few years, the identification of cellular sources of the six known hormones of the anterior pituitary has been established in the rat and mouse.
Methods for determination of endocrine function of each cell type distinguished by electron microscopy are discussed in detail. Some of them have been fully utilized by electron microscopists who are engaged in functional morphology of the anterior pituitary gland, but a few remaining ones are still in the trial stage.
The nomenclature of the anterior pituitary cells indicating the morphological characteristics is now replaced by a new terms implying the function, namely indicating the hormone which is secreted by a given cell type.
As the rats have been most frequently observed and best known as to function, the description of this paper is concentrated to the rat anterior pituitary. The electron microscopical features of each of the seven cell types in either normal, pathological or experimental condition are described, and some representative electron micrographs are illustrated.

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The thyroid gland of normal adult specimens of teleosts, the Semicossyphus reticulatus and the Sebastiscus marmoratus obtained in the Inland Sea of Seto in June, were observed with the electron microscope.
The fine structure of the thyroid follicle cell is generally more similar to that of the higher vertebrates than to that of the cyclostome such as a lamprey in the cytoplasmic structures especially in the well developed rough-surfaced endoplasmic reticulum and numerous intracellular granules and droplets, though the classification of the granules and droplets is not so easy as those of higher vertebrates. The characteristic of the follicular cell is the occurrence of aggregates of filamentous structures, myelin bodies, small dense bodies, vesicular structures, and irregularly shaped dense substances in large colloid droplets. They might be formed by fusion of the altered colloid droplet and cytolysome.
The capillary endothelial cells show a pored structure as those of the higher vertebrate, though those of the Seriola quinqueradiata and Anguilla japonica have been reported to be non-pored. In the endothelial cell of the capillary and the arteriole of the thyroid of the Sebastiscus marmoratus, small groups of fine filaments were seen.

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Electron microscopic observation was made on the changes in the cerebral cortex of the newborn and developing rat brain caused by dehydration. Rats ranging in age from one to four days were kept unfed for from 24 to 96 hours.
1. In the dehydrated animals, the brain capillaries became irregular in outline and the endothelial cells swelled with an increased cytoplasmic density. Vacuole formation in the endothelial cytoplasm became conspicuous and the basement membrane increased in width. Disappearance of the thin fold which overlaps endothelial cells may be recognized. The pericapillary glial processes may be destructed. These changes appear to represent a progressive reaction to the dehydration and to indicate an increased permeability in the blood brain barrier. As the dehydration goes on, these changes become more conspicuous, but in severely affected cases, swollen endothelial cells gradually lose their density with a decrease in the number of organelles.
2. The most evident and constant changes in the nerve cells are vacuolar cisternae of the rough surfaced endoplasmic reticulum, derangement of the accumulations of the reticulum cisternae as Nissl bodies and increased vesicles in the cytoplasm. Mitochondrial degeneration is not constant. These cytologic damages are recognizable in all dehydrated animals though varying considerably in degree. The vacuole formation becomes more conspicuous as the dehydration goes on.
3. Oligodendroglial cells show considerable changes such as vesiculation and vacuolation of the cytoplasm.
4. No remarkable changes of intracytoplasmic organelles occur in astrocyte, but shrinkage of the cell body is observed occasionally.
5. In general, cytologic changes due to dehydration are observed more frequently and strongly around the capillary.

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The recent works of the present author have revealed that, at least in the cat, the excitatory and inhibitory synapses are clearly distinguished by the shape and size of the synaptic vesicles contained in them. A comparative consideration of the electrophysiological and electron-microscopical findings has namely led him to the hypothesis that the excitatory synapses contain spheroid synaptic vesicles of 500Å in diameter (S-type) whereas the inhibitory synapses contain flattened ones of a smaller size (F-type).
The application of this hypothesis in identifying the different kinds of nerve fibers in the cerebellar cortex of the cat gives the following results:
1. The axon terminals of the Purkinje cells, which are electrophysiologically known as inhibitory neurons, are represented by the synapses of F-type containing flattened vesicles. These fibers are characterized by a conspicuous lamellar structure of the axoplasm in the course.
2. The granule cells are regarded as excitatory neurons, as their axons or parallel fibers contain in their terminals synaptic vesicles of the S-type.
3. The stellate cells are indicated to be of an inhibitory nature as their axon terminals contain F-type synaptic vesicles.
4. The basket cells are inhibitory neurons whose terminal axons embrace the Purkinje cell somata and contain F-type vesicles in their synapses.
5. The axon terminals of the Golgi cells are involved, together with the dendrites of the granule cells and with the axon terminals of the mossy fibers, in the formation of the cerebellar glomeruli. Their synapses contain F-type vesicles indicating the inhibitory nature of these neurons.
6. The climbing fibers go up parallel with the primary or secondary dendrites of the Purkinje cells, making synapses on the short-necked spines of the Purkinje cell dendrites. These synapses contain S-type vesicles in correspondence with the excitatory nature of these fibers established electrophysiologically.
7. The mossy fibers, after entering the granule layer abruptly lose their myelin and swell up in huge synapses. The synaptic vesicles of the S-type contained there indicate the excitatory nature of the fibers.

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