The parafollicular cells of the thyroid gland of the dog were studied by electron microscopy, and the following results were obtained.
Parafollicular cells were larger than ordinary follicle cells and have large round to oval, sometimes irregular-shaped nuclei. They were located on the follicle walls or wedged in between follicle cells but usually did not border the follicle cavity directly. They were round to oval, but when in groups they were polygonal and gathered together as buds and plates surrounded in a set with the follicles by argyrophilic fibers and capillaries. The parafollicular cells could be classified into two kinds, clear and dark cells. The latter were found to be less in number. Thyroid stimulating hormone produced no significant changeno increase of colloid dropletsin them.
At the electron microscopic level, the parafollicular cells showed the same findings as by light microscopy, but revealed the following fine structure.
1. The majority of parafollicular cells were less electron dense in the cytoplasm than follicle cells.2. Rough-surfaced endoplasmic reticulum was poorly developed and dispersed in small numbers in the form of small vesicles and sometimes as a few thin lamellar forms together with the vesicular forms. This was one of the characteristics of this kind of cell.3. The mitochondria were smaller in size and number than of follicle cells. They showed round to oval, partly branched profiles with ruffled outline, and had irregularly arranged cristae mitochondriales.4. The Golg is complex was well-developed and consisted of thin lamellar sacs, vesicles and vacuoles.5. Secretory granules were found in small numbers and different in size and electron density. They were small near the Golg is complex, probably taking their origin in the complex.6. No desmosomes nor complicated interdigitations were found between themselves or in between them and the follicle cell.7. Thyroid stimulating hormone produced no significant change in their cytoplasm, also at the electron microscopic level.8. In both the control series and experimental series no intermediate forms between the follicle cells and parafollicular cells appeared.9. The parafollicular cells may be considered to be an independent cell species different in origin, structure and function from the follicle cells.

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The radial components of the myelin sheath in the optic nerve of frogs (Rana pipiens) have been studied by electron mincroscopy. The nerves were fixed in OsO₄ or KMnO₄, embedded in methacrylate or epoxy resin, and, after sectionings, non-stained or stained with either potassium permanganate or plumbite ion method. The results obtained are summarized as follows:
1. The radial components are present in some of the myelin sheath of the optic nerve of the frog, Rana pipiens.
2. The radial components in transverse sections of the myelin lamellar layers appear as a series of small thickenings which are present in the interperiod lines of the myelin lamellae and stand in several rows arranged radially across the thickness of the myelin sheath. The dense period line of the myelin sheath at the site of the radial component somewhat increases the electron density. In oblique sections of the myelin lamellae the radial components appear as dense lines which show a parallel array running across the thickness of the sheath.
3. The thickness of the myelin lamellae increases at the site of the radial components.
4. The rows of the thickenings in the radial components show two different patterns of distribution in the myelin sheath; the diffuse and condensed types. The former is scattered in a relatively large part of the myelin sheath and shows a fairly large interval of 500 to 600Å between the individual rows, while the latter occurs in a small limited area of the sheath in group of many parallel rows showing a relatively small interval of 150 to 270Åbetween each rows.
5. Most of the rows of thickenings extend throughout the thickness of the myelin sheath, while some of them extend for only a part of the superficial layers of the myelin lamellae.
6. Sometimes the rows of the radial components extend continuously over two adjacent myelin sheaths which come into intimate contact side by side with each other.

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Young adult male albino rats were exposed to 600 r of total-body X-irradiation and the occurrence of plasma cells was studied on sections made from thymus, mesenteric lymph nodes and spleen. One to 30 days after irradiation, the marked increase in percentage of plasma cells has occurred, reaching its peak during the period of three to 10 days, decreasing in number gradually thereafter. In a quantitative estimation of plasma cell on the 10th day after irradiation, by means of cell counting method used in thymolymphatic tissues (Awaya,1962) and of differential cell counts in imprint, there was found an absolute increase of these cells in all organs with the exception of thymus in spite of a marked decrease of total number of nucleated cells.

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The intraperitoneal injection of homogenate, nuclear or cytoplasmic fractions of lymphoid cells into other rats of the same strain causes a conspicuous plasma cell proliferation in host lymphatic tissues. This suggests a possibility that a product of destroyed lymphocytes might promote the plasma-cellular reaction.

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Various dye fluids were injected into the blood vessels of frogs and rabbits and observationsin vivo were made of their patterns of extravasation.
(1) A relation was noted between the site and mode of extravasation and the diffusibility of dyediffusibility itself being related to the size of dye particles.
(2) Following intravenous injection the parts of blood vessels, excepting arterioles, where extravasation can occur were similar with both highly and poorly diffusible dyes. These parts extended from the arterioles to the venules. But the sequence of sites of extravasation and their frequency were in a reverse relationship. With highly diffusible dyes extravasation occurred in the sequence of precapillaries, capillaries, postcapillaries, venules and arterioles, and along the entire course of these vessels, so that the spread of extravasated dyes produced a color band form with the vessels along the long axis of the band. Contrary to this, with poorly diffusible dyes extravasation occurred first from the postcapillaires and venules, especially at the transitional portion of the two, then from the capillaries, and belatedly sometimes from the precapillaries. The mode of extravasation was sporadic and solitary, so that the spread of dye was oval in shape with the vessel running through the axis of the oval.
(3) Following intra-arterial injection of highly diffusible dyes leakage of dye occurred in the sequence of arterioles, precapillaries, capillaries and postcapillaries, but not from the venules. With poorly diffusible dyes the sequence was capillaries, pre- and post-capillaries and venules. No leakage occurred from the arterioles. Thus, following intra-arterial injection of any type of dye, extravasation tended to occur in general from the arterial side, when compared with intravenous injection. However, the modes of spread were similar to those following intravenous injection, and highly diffusible dyes escaped from all over the vessels concerned forming a color band, and poorly diffusible dyes sporadically, from here and there, producing oval shaped forms.
(4) There was no marked difference in the sites and modes of extravasation between frogs and rabbits.
(5) Important factors determing the sites and modes of extravasation were believed to be the size of dye particles and the differences in structure of blood vessel walls according to locality (probably such as the grades in ease of disconnection of the junction between endothelial cells). However, when the local blood flow stops extravasation practically stops, so that the local blood pressure can also be said to promote extravasation. Various physical, chemical, physicochemical and biological factors that influence permeability are also believed to produce changes in size of particles and in structure of blood vessel walls.

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Of the mitochondria in human hepatic cells obtained from various cases of diseases, the specific modified findings were as follows:
(1) On light optics, those which show a high stainability thick, long rod shaped or spindle shaped. They are observed in a comparatively large number in the cells in the perilobular area of the liver.
(2) On electron optics, those which show an extreme swelling some reaching 1.5μ to 4.0, μ in the long axis. They show an unclear limiting membrane and cristae. Some of them show a destruction of the limiting membrane, a defect of cristae, coagulation in the mitochondrial matrix, revealing various findings. Such findings are considered to correspond to those on light optics.
(3) In light and electron optics, the various modified findings of mitochondria seem to be subjected to a kind of degeneration.
(4) The various modified finding of mitochondria do not selectively appear in the materials obtained from various cases of diseases. Still, they seem to appear rather frequently in severe cases.

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Thyroid glandsfollicle cells of lower vertebrates, namely of the pigeons and tortoises were electron microscopically observed in relation to the endocrine mechanism involved in the secretion, storage, and release of thyroid hormone, and the results obtained were discussed by comparing with data known of mammalian thyroid gland.
1. The follicle cells examined here possess essentially almost the same structure as those known in mammals. Probably almost the same processes of function occur in almost the same manner in each organelle.
2. From the apical plasma membrane, a number of microvilli project into the colloid, and their matrix has usually no special structure such as vesicles, vacuoles, and secretory granules.
3. The apical plasma membrane found between the microvilli shows sometimes tiny indentations. The indentations are more frequent in the tortoise.
4. In the superficial cytoplasmic zone are more or less extensively seen small vesicles which remain impossible to determine, whether they are related to the indentations or to the vesicles of the Golgi apparatus.
5. Along the apical margins of the lateral plasma membranes of adjacent follicle cells, well-developed terminal bars are found, and in the other part interdigitations and desmosomes are formed. In the pigeon the interdigitations are simply peg-like and desmosomes also poorly developed. In the tortoise the interdigitations are far more complicated, especially in places where more than two follicle cells are in contact. The interdigitations appear as severallayered concentric circles, semicircles and whorles. Desmosomes of the tortoise are well-developed and divisible into three types.
6. The mitochondria seen in both animals are almost the same in structure as those reported in mammals.
7. The Golgi apparatus consists of numerous vesicles, a number of vacuoles and several arcuated lamellae, and appears to be the site of producing of hormone substance as secretory granules.
8. The secretory granules are of different electron density and size, generally small near the Golgi apparatus, and as they move towards the apical plasma membrane increase in size and density. In the superficial cytoplasmic zone, they are again of low density. No morphological evidence of secretion as such into the follicle cavity is observed.
9. The rough-surfaced endoplasmic reticulum is well-developed, containing homogeneous substance like the follicle colloid, and appears as sacs and cisternae of different number, size and shape. Some of them round up and resemble in shape ordinary secretory granules. In the basal half, the sacs and cisternae dilate to a considerable degree and sometimes occupy its entire extent. This is exaggerated in the pigeon.
10. In the pigeon, paired centrioles are found in the apical half of the cell body and one of them serves as the point of origin of the central flagellum.
11. The basal plasma membrane is smooth, but sometimes in the tortoise infoldings are formed. The basement membrane is seen beneath the basal plasma membrane and in the tortoise is sometimes thicker containing fibrillar elements.
12. The nuclei show no special characteristics in both animals.

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