A histochemical study was made on the succinic dehydrogenase activity of the M. gastrocnemius, M. soleus, M. quadriceps femoris, M. biceps femoris, M. masseter, M. temporalis and M. digastricus in the rat, rabbit and cat. According to the intensity of the succinic dehydrogenase activity, the muscle fibers were divided into three types, i. e. white fibers with a low, red fibers with a high, and medium fibers with an intermediate enzymatic activity.
Every muscle studied was composed of two or three types of muscle fiber, and never of a single type of fiber. The ratio in number of muscle fibers of three types varied in different muscles. The limb muscles contained white and medium fibers in higher percentage, while the jaw muscles composed mainly of red and medium muscle fibers in these three animals.
White fibers were generally more numerous in the superficial part of the muscle, whereas red and medium fibers in the axial or deep portion of the muscle.

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The fine structure of human ductuli efferentes in fetal and adult epididymis was studied with the electron microscope.
The epithelium of the ductuli efferentes is composed of columnal cells of both ciliated and non-ciliated types.
The morphology of the ciliated cells do not differ significantly between fetal and adult cells. In the supranuclear zone, lysosomes and multivesicular bodies are recognized. In addition, adult cells show waste pigment granules and occasionally a lamellated granular endoplasmic reticulum.
The fine structure of the non-ciliated cells suggests their function in absorption and secretion. Based on the morphological differences in their apical cell surface, the adult non-ciliated cells can be classified into 3 types. It was assumed that type I cells possess absorptive function and those of type II and III contribute to the secretion.
Morphological differences by age in the adult cells are hardly recognizable except for senile degeneration.

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The remnant liver tissues of the adult male rats after partial hepatectomy by the method of HIGGINS and ANDERSON were studied by electron microscopy, Particular attention was paid to the mechanism of the formation of fatty liver from the viewpoint of morphology.
During first ten hours after partial hepatectomy, the hepatic parenchymal cells showed various degenerating processes in the cytoplasm. Mitochondria were swollen and often deformed. Glycogen particles were remarkably reduced and at last disappeared from the cytoplasmic matrix, whereas agranular endoplasmic reticulum was increased to develope. There appeared to be some intimate reciprocal relationships between the depletion of glycogen and the increase of agranular endoplasmic reticulum. At this stage, however, no lipid particles appeared in the cytoplasm.
Fifteen hours after hepatectomy, glycogen particles disappeared completely from the cytoplasm. Instead, lipid droplets began to deposit in the cytoplasmic matrix in close association with agranular endoplasmic reticula.
On the other hand, the cytoplasm close to the space of DISSE showed the prominent increase of pinocytotic vesicles, the cavity of which contained small dense particles, presumably mobilized fat particles. Similar dense particles were also observed confined to agranular vesicles in the cytoplasm apart from the cell surface, where dense particles tended to develope into large droplets by coalescence, and at last to lose their limiting membranes. All of these were assumed to represent the process linked with each other, to which lipid depositions are at least in part attributed.
Twenty hours after hepatectomy, the maximum deposition of lipid in the cytoplasm was observed and the peripheral region of cytoplasm contained much more lipid than the central. Most parts of the cytoplasm were occupied with well-developed agranular endoplasmic reticulum.
Two days after operation, or thereafter, glycogen particles again began to appear in the cytoplasm. Instead, at the same time, the lipid droplets were gradually decreased and finally disappeared from the cytoplasm.
Five to ten days after operation, the parenchymal cells did not show any changes in fine structure but returned to the appearance of normal cells.
From the present study, it is assumed that in the remnant hepatic parenchymal cells after partial hepatectomy, glycogen particles are promptly consumed for increased need of energy supply owing to high metabolic rate, and that, in following the depletion of glycogen, fats are mobilized as energy source to the liver from peripheral depots.
The present study suggests the possibility that the fatty liver after partial hepatectomy is in part induced by the metabolic transformation from glycogen to lipids, and in part by the mobilization of lipids to the liver from the peripheral depots. It is in any way apparent that formation of glycogen and fat occurs in close association with agranular endoplasmic reticulum.

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The gastric chief cells of the hibernating bats (Rhinolophus ferrum-equinum nippon) were observed with the electron microscope. The chief cells contain an extensive granular endoplasmic reticulum and many mitochondria. In the supranuclear region a well developed Golgi apparatus and various number of secretory granules (zymogen granules) are seen. Many microvilli protrude into the lumen from the apical surface of cell. The intercellular interdigitations on the lateral cell border is poorly formed. Near the luminal end of the lateral cell border there occurs“junctional complex” which is composed of zonula occludens, zonula adhaerens and macula adhaerens (desmosome). Besides, several desmosomes are found also in deeper part of the cell border. Basal infoldings are scarcely seen at the cell base. In the matrix of the microvilli a number of electron opaque filaments run along the long axis of them, to be continued with a faint filamentous structure in the apical cytoplasm subjacent the luminal cell surface.
The unit membrane structure (triple-layered structure) of the plasma membrane is clearly identified both in the apical and the lateral cell boundaries, but it is rather indistinct at the cell base. The thickness of each layer of the plasma membrane varies to some extent in different regions; the inner dense layer gives fairly constant value of about 30Å, while the outer dense layer shows a relatively variable thickness in various regions. In the apical and microvillous region, each of the three layers of the plasma membrane measures about 30Å in thickness. The triple-layered membrane can be identified also in Golgi apparatus and mitochondria, whereas it is rather difficult to discern the three layers in the limiting membrane of the granular endoplasmic reticulum.
The secretory granules are divided into two types in regard to their limiting membrane: One is covered by the triple-layered membrane (85Å), and the other by a thinner monolayered membrane (40Å). The majority of the mature secretory granules and the small immature ones found in the vicinity of the Golgi apparatus are covered by the triple-layered membrane, while the monolayered one covers as a rule only some of larger secretory granules found in the apical cytoplasm. The triplelayered limiting membrane may be transformed into the monolayered during the process of maturing of the secretory granules.
The matrix of the secretory granules has a moderate electron density and a homogeneous compact texture. The small immature granules mentioned above are relatively opaque showing a somewhat coarse texture. The large, electron lucent granules which are found in the apical cytoplasm and show sometimes a coarse texture are regarded as highly matured secretory granules or as secretory vacuoles.
Two modes of secretory granule extrusion have been confirmed in the gastric chief cells of the bat. In the first case, the secretory granules are extruded into glandular lumen by means of reversed pinocytosis; in the second, the electron lucent content of matured secretory granules in the apical cytoplasm probably either permeates through the altered or monolayered limiting membrane, or flow out through the interrupted portion of the limiting membrane into the surrounding cytoplasm, and thereafter the secretion may be emptied by diffusion through the plasma membrane into the glandular lumen. In the last stage of this extrusion process it may be possible that the secretion remaining in the apical cytoplasm is taken up in smooth surfaced vesicles scattered around there to be later released by reversed pinocytosis into the lumen.
In the bats secretory function of the gastric chief cells seems to continue to some extent even in the hibernating period. The intermediary or prosecretory granules are found among the Golgi vacuoles and the immature secretory granules and show intermediate morphological characteristics between the both.

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The author studied the cytokinetics in the early postnatal histogenesis of mouse brain by autoradiography after repeated injections of tritiated thymidine.
1. The cellular proliferation is especially remarkable in the external granular layer of the cerebellum and in the subependymal layer of the cerebrum. The proliferative activity of the cells in the external granular layer is the highest in the first postnatal week, then decreases rapidly with thinning of this layer and is lost within 20 days of age. The proliferative activity of the cells in the subependymal layer is remarkable for a week or two after birth followed by gradual decrease thereafter, but the cellular multiplication can be noticed even in the vestigial subependymal layer of adult mouse cerebrum.
2. The external granular layer is divided from the functional point of view into two zones: outer zone composed of actively proliferating cells and inner zone consisting of cells which have lost the proliferative activity. The author named the former “germinal zone” and the latter “transitional zone”.
3. The generation time and duration of each phase in the mitotic cycle of the germinal cell in the external granular layer and of immature cell in the subependymal layer were calculated by the cumulative labeling method. The t_G, t₁, t_S, t₂ and t_M of the germinal cells of the mice between 1 to 10 days of life were 15 to 29, 7 to 20, 5.6 to 8.0 and 1 to 2 hours and 0.4 to 0.7 hour, respectively and those of the immature cells in the subependymal layer of 1 and 3-day-old mice were 63 to 65, 52.6 to 54.6, 7 to 10 and 1 to 2 hours and 0.4 hour, respectively.
4. Almost all of the transitional cells in the external granular layer migrate through the molecular layer into the internal granular layer to mature into granule cell neurons; and even after 10 days of age, about 48 to 49per cent of the cells in the internal granular layer are provided from the external granular layer.
5. Many of immature cells in the subependymal layer migrate into the olfactory lobe. Besides the olfactory lobe, the corpus callosum, internal and external capsules and other fiber tracts depend on the subependymal layer for the abundant supply of cells. Some of the cells which have migrated into olfactory lobe differentiate into the granule cells, whereas the majority of the immature cells in the subependymal layer differentiate into the oligodendrocytes and the astrocytes.

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