Besides some findings concerning the microscopic anatomy of the pancreatic islets and the histological peculiarities of the neuro-insular complexes in the garden dormouse, Eliomys quercinus, this study confirms the existence of islet cells of three different types on the basis of their distribution, cytological characteristics and their changes during the annual cycle.
The B-cells are located in the central region of the islets and represent about 70% of the total cell population. Their secretory granules are relatively small and stainable with paraldehyde-fuchsin and other basic dyes after permanganate oxidation. They are rich in protein-bound disulphide groups and devoid of histochemically detectable indoles.
The A-cells are of greater size than the two other cell types. The abundant secretory granules of these cells remain erythrophilic after permanganate oxidation. Devoid of disulphide groups, they are positive in histochemically detectable indoles. This cell type is located near the border of the islets and fails to show any argyrophilia after impregnation following the methods of Davenport and Grimelius.
The D-cells are represented by argyrophilic elements of conical or pear-shape. Their cytoplasmic processes are oriented toward the capillaries, the cell bodies lying at the periphery of the islet tissue. The secretory product shows a metachromatic reaction with toluidine blue which is enhanced by hydrochloric hydrolysis. This product contains neither histochemically detectable indoles nor disulphides. It exhibits cyanophilia after permanganate oxidation. The number of D-cells and the abundance of the argyrophilic secretory product undergo a characteristic change during the annual cycle. A progressive decrease from January to June is followed by a progressive increase. The maximum amount of cells and of argyrophilic substance is encountered in prehibernating animals. This type of change differs from the annual cycle of B- and A-cells. Together with the lack of any transitional forms between D-cells and the other cell types it is consistent with the conception of the independence of the three cell types. The annual cycle of D-cells is not correlated to the changes in blood sugar and hepatic glycogen, but is closely parallel to changes in fatty acids in the liver parenchyma and blood plasma suggesting a possible elaboration by this cell type of substances which regulate lipid metabolism.

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In the experimental pneumonitis of rabbits induced by the agricultural chemical, Blasticidin-S, regeneration of the alveoli was observed by light and electron microscopy. The animals were singly injected with BC-S solution through the trachea into the alveoli and sacrificed at certain intervals. The non-ciliated epithelial cells at the terminal portion of the respiratory bronchiole began to proliferate three days following BC-S administration. These proliferated epithelial cells extruded outward to form glandular structures five and six days after the treatment. These structures increased in number and blood capillaries developed beneath their epithelia seven and eight days after the treatment. The epithelial cells lining the glandular structures began to differentiate cytologically after nine and ten days, forming both types of alveolar cells, i.e., great and squamous alveolar epithelial cells twelve and fourteen days after the treatment. Blood-air barriers were accomplished by this stage. After the ninth day the epithelial cells, proliferating and branching off, began to form new alveoli also within the destroyed masses of the pulmonary tissues.
In this experimental study it was concluded that the regeneration of alveoli originates from the non-ciliated epithelial cells in the terminal portion of the respiratory bronchioles. These cells seemed to correspond to the special non-ciliated cells found in the normal rabbit which differed in fine structure from ordinary non-ciliated cells, Clara cells, and were supposed to be cells of an undifferentiated type. The detailed processes of the differentiation of both types of alveolar epithelial cells were described and discussed.

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The gut epithelium of larval and juvenile carp is composed principally of columnar cells and intermingled goblet cells. In the mid gut of juveniles so-called pear-shaped cells are occasionally found among the columnar epithelial cells.
On the basis of absorption patterns, the gut of larvae or juveniles examined after feeding can be divided into two major portions: the mid gut and the posterior gut. In the mid gut, columnar epithelial cells are crowded with fairly large fat droplets. During fat absorption the microvilli and terminal web of the columnar cells appear normal, and no pinocytotic invaginations are seen. On the other hand, in the posterior gut, the columnar epithelial cells are loaded with peculiar vacuoles containing electron lucent materials related to ingested protein. In these cells, the apical cytoplasm is characterized by vigorous pinocytotic invaginations, small vesicles, tubules, and bundles of fine filaments, all associated with the vacuoles.
Electron microscopic observations revealed that fat and protein are absorbed by different pathways in teteost larvae. The mid gut appears to be mainly involved in fat absorption without a process of pinocytosis, and the posterior gut is mainly implicated in protein absorption by a process of pinocytosis.

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The cytoplasm of PAS cells in X-irradiated mice thymus was studied with autoradiographical and histochemical technics. Tritiated thymidine was incorporated into the cytoplasmic areas of PAS cells where a positive Feulgen's nuclear reaction was observed. The intensity of the nuclear reaction varied greatly; some cells were stained homogeneously whereas others showed the existence of granular DNA component in their cytoplasm. These cytoplasmic DNA components are probably derived from phagocytized thymocytes. Although their fate is unknown, their utilization by surrounding lymphocytes is most likely.

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In human thymuses obtained from thoracic surgery myoid cells were studied by light and electron microscopy. The myoid cells were found, though not abundantly, in 19 out of 21 thymuses examined. They should thus probably be considered as common elements in the human thymus. In light microscopy the myoid cells appeared as a large, round or somewhat elongate cells with markedly eosinophilic cytoplasm and a vesicular, irregularly-shaped nucleus. The cells were usually found singly in the medulla, especially near the cortico-medullary boundary. Electron microscopy revealed that the myoid cells were characterized by the presence of myofibrils and triad-like membranous structures. These characteristic cytoplasmic components were the same in ultrastructural features as those known in striated muscle cells, although they were less developed and far less regularly disposed in the myoid cells than in muscle cells. In other cytological features, the myoid cells were similar to the usual epithelial reticular cells. In addition, small poorly-developed desmosomes were occasionally encountered between the myoid cells and adjacent epithelial reticular cells. From these findings the myoid cells may be considered a form of epithelial reticular cell.

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Acetone-dehydrated and air-dried spread specimens of mitochondria isolated from the liver, kidney and skeletal muscle of the rat were observed under the scanning electron microscope.
The mitochondria from the liver differed in shape according to the concentrations of sucrose used in preparation. They were of ovoid and rod-like shape when 0.44M sucrose was used. With 0.32M sucrose they resembled either rolled larvae or doughnuts. With 0.25M sucrose spherical forms were predominant. Processes of rolling up of the original rod shape into a ring and its swelling into a spherical form in hypotonic media were suggested.
The mitochondria from the kidney and muscle were less uniform in shape and size than those from the liver.

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