The pineal body of the mouse was studied by electron microscopy especially as to whether this organ is secretory in nature. The cytoplasmic processes of the pinealocytes were frequently observed together with the axons around the blood capillaries. The pinealocyte processes contain variable amounts of vesicles of various sizes and appearances. Large vesicles are of two kinds: granular (cored) and agranular (clear). The granular vesicles measure a mean diameter of 1160Å. Small vesicles contain no electron dense core, resembling those in axon terminals in size and shape. It is most likely that the large granular vesicles are formed in the Golgi apparatus in the cell bodies and transported to the cell processes. By a morphological analogy to the secretory vesicles of the neurosecretory cells the large granular vesicles would be considered as the secretory material of the pinealocytes. The nerve axons in the perivascular spaces are loaded with numerous small, granular and agranular vesicles which measure approximately 600Å in diameter. They are considered sympathetic and adrenergic in nature.
The endothelial cells of the capillaries which are surrounded by the tissue spaces are attenuated and possess numerous fenestrations with an average diameter of 650Å. The occurrence of the fenestrations may also suggest the possible secretory nature of the pineal body.

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The epithelial cells of the midgut and hindgut in the crayfish were studied by electron microscopy. The epithelial cells of the midgut are characterized by formation of cytoplasmic tubules or lamellae in the basal half of the cells. These structures appear to be specified infoldings of lateral plasma membranes, and are assumed to be similar in terms of structure and function to the cytoplasmic tubular structures in the chloride cell of fish gill filaments. In the hindgut, the epithelial cells are prominently provided with cytoplasmic microtubules. Thick bundles of them are attached to both luminal and basal plasma membranes. When attached with microtubules, the basal plasma membrane of the epithelial cells usually makes an epithelio-myonal junction with that of the processes of striated muscle fibers derived from the muscular coat, in a manner similar to that of the intercalated disc of the cardiac muscles.

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The spinal cord, especially the thoracic and sacral lateral horn cells and their fiber connections of the dog were studied with the fluorescence method for catecholamine developed by FALCK and HILLARP. The thoracic and sacral rhizotomy, and the treatment with drugs that interfere with catecholamine metabolism, were also performed in order to clarify the possibility of catecholamine-biosynthesis in the preganglionic neurons.
The thoracic and sacral lateral horn cells are unable to produce or take up catecholamines. The thoracic preganglionic neurons are bilaterally innervated by monoaminergic nerve fibers originating from some supraspinal locus; the fibers descending mainly in the lateral funiculus. The sacral parasympathetic nuclei are also similarly innervated with the monoaminergic fibers. Several unmyelinated fluorescent fibers originating from supraspinal locus descend in the lateral funiculus, and appear in the anterior roots of the thoracic and sacral segments without any relays on the interneurons in the spinal cord.

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Electron microscopic studies were made of thyroid glands from 10-30 day-hypophysectomized rats, 20-21 day-hypophysectomized rats treated with TSH 30min-4hrs before sacrifice, and 20-21 day-hypophysectomized rats treated with actinomycin D 4-10hrs before sacrifice and with TSH 30min-4hrs prior to fixation.
1. In the hypophysectomized rats the thyroid follicular cells were markedly attenuated and conspicuously reduced in size of their rough endoplasmic reticulum and Golgi apparatus. Most cytoplasmic small vesicles and colloid droplets had disappeared except for a few lysosome-like dense bodies. No fine structural changes were recognized in the parafollicular cell after hypophysectomy.
2. Some elements of the rough endoplasmic reticulum showed simple (but not reticular) enlarged cisternal structures and the small vesicles were increased markedly 2-4 hours after injection of TSH in the hypophysectomized rats, while in the hypophysectomized animals treated with actinomycin D the rough endoplasmic reticulum remains flattended and small vesicles were smaller in number after TSH injection.
3. Cytoplasmic large colloid droplets appeared in the hypophysectomized thyroid cell 30min after injection of TSH, and were increased in number with time. However, these droplets were hardly recognized in the rat treated with actinomycin D and TSH. Actinomycin D is considered to inhibit the endocytic activity of the follicular cell induced by TSH-injection; the possible mechanisms of this effect were discussed.

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The pars intermedia of the pituitary of the human fetuses aged from 5 to 9 months was studied with the electron microscope. The pars intermedia consists of epithelial cells bounding the Rathke's hypophysial cleft and an underlying interstitial space containing blood capillaries, collagen fibrils and some fibrocytes. Two kinds of epithelial cells are distinguishable: the marginal cells which line the cleft and the glandular cells which are located under the former and contain secretory granules of various sizes.
The marginal cells are columnar and characterized by the presence of a large number of glycogen granules which tend to decrease in number with advancing age. The cells of this kind show no dense granules suggesting secretion.
The glandular cells are composed of three types: Type I cells contain granules about 2, 000Å in diameter, and type II cells larger ones about 3, 500Å. Both types of cells are commonly observed and have a well-developed Golgi apparatus and rough endoplasmic reticulum besides numerons secretory granules. The cells of type III possess small granules about 1, 200Å and are few in number.
Many of the capillary endothelial cells found in the interstitial space show fenestrations of the cytoplasm as early as the fifth month of fetal life.
Possible activity of the pars intermedia in hormonal secretion in this early stage of development was discussed.

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The dog retina was investigated with the fluorescence microscopical method for catecholamines introduced by FALCK and his co-workers (1962).
Dopamine was administered intravenously, intraventricularly and intravitreously. Both intravenous and intraventricular dopamine significantly increased the number of the fluorescent amacrine cells. With the intravitreous administration almost all amacrine cells became fluorescent. These facts indicate that at least a large part of the amacrine cells is dopaminergic by nature. On the other hand, no significant changes were recognized in the fluorescent elements other than those in the amacrine cells.
Dopamine intravitreously administered entered into the stratum cerebrale, but not beyond this. It is supposed that there is a certain barrier between the stratum cerebrale and neuroepitheliale.

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The thyroid glands of the dog, cat, rabbit, rat, mouse, and guinea pig were stained by a modification of the silver impregnation method of DAVENPORT (1930). The parafollicular cells were selectively demonstrated with their cytoplasm filled with fine, silver blackened granules. As fixative Bouin, Carnoy or 10% formalin could be used for this method. Because of its simple and quick procedure and better fixation, this impregnation method has advantages over that of Cajal which has been used for the demonstration of this type of cell.
Puppies were given three injections (2.5hr intervals) of 50mg/kg CaCl₂ and 100, 000 IU Vitamine D₂. The parafollicular cells after this treatment showed a marked decrease in their argyrophil granules and a strong shift of the granules to the basal side of cell. Some cells even completely lost their granules and could be identified as parafollicular cells by bleaching the impregnated section and restaining it with hematoxylin-eosin which revealed the nuclei characteristic of this type of cell.
The relative population of the parafollicular cells to follicular cells was calculated and it became obvious that they were by far more numerous than ever estimated (e. g., 30-90 parafollicular cells per 100 follicular cells in the dog and 30 parafollicular cells in the rat).
As to the problem of the origin of the parafollicular cells, the hypothesis that these cells were derived from the ultimobranchial body was considered. The ciliated epithelium of the ultimobranchial cyst in the thyroid of postnatal animals, however, contained few argyrophil cells.

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The dog carotid body was studied by light and electron microscopy. It consisted of small clumps of glomus cells, interspersed between blood vessels. The glomus cells were classified into chromaffin and nonchromaffin cells, on the basis of their light microscopic reaction to potassium dichromate. The chromaffin cells were more infrequent in occurrence and their cytoplasm was filled with intensely osmiophilic granules of various sizes and irregular shapes, while the nonchromaffin cells were predominant in number and their specific granules were more uniform in structure and showed moderate osmiophilia. It was remarked that the chromaffin and nonchromaffin cells of the carotid body respectively correspond in their fine structure to the noradrenalin and adrenalin storing cells of the adrenal medulla, and the possible secretory function of the glomus cells was discussed. The light microscopically “negative” chromaffin reaction in a part of glomus cells may not necessarily indicate the absence of catecholamines but a low value of them in these cells.
Nerve endings occurred frequently on the glomus cells. They resemble in fine structure the nerve endings to the adrenal medullary cells. The synaptic vesicles were present on the neural side of the synapse and it was concluded that the glomus cells receive an efferent innervation. Besides the ordinary spherical, synaptic vesicles of flattened shape were recognized which may possibly be of an inhibitory nature, if the hypothesis of UCHIZONO be applicable in the glomus innervation.
Ganglion cells were found sporadically, in the interlobular connective tissue.

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