The parotid gland of the Mongolian gerbil, Meriones meridianus, was examined by the electron microscopy. A peculiar feature of the serous acinar cells is that mature secretory granules have a bipartite substructure consisting of a centrally-located large dense homogeneous core and the peripheral rim of a less dense, fibrillo-granular matrix. The latter is histochemically revealed to be rich in acid mucosubstance while the former may be mainly composed of zymogenic materials because of its dense homogeneous texture. Such a heterogeneity of granule content is detectable in all the materials examined, regardless of fixation and staining methods and of animal age. Another characteristic feature of the gland is observable in the duct cells, which provide the basal hemidesmosomes attached with microfilaments and the network system of agranular cytoplasmic tubules, closely related to the basal plasma membrane. Functional significance of these structures is discussed.

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The development and sequence of eruption of the maxillary cheek-teeth of rabbits were studied by histological methods.
The presence of three deciduous molars which were replaced by correspondent premolars and of three permanent molars without predecessors was confirmed. The eruption of the maxillary deciduous molars was shown to begin at 4 days postnatally and that of the permanent molar at 9 days, while the eruption of the premolars occurs from 24 days on, replacing the deciduous molars which are exfoliated. The last tooth to erupt is the M³. At 32 days all the permanent cheek-teeth are erupted.
The deciduous molars are completely developed at birth, root resorption starting at 4 days.
On the first day the premolars are in the bell stage and in the M¹ and M² amelogenesis is taking place.
After 27 days the development of the permanent maxillary cheek-teeth is completed. Dentinogenesis, amelogenesis and cementogenesis were observed in all of them.

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Prolonged administration of cycloheximide, an inhibitor of ribosomal protein synthesis, induces degeneration of rabbit taste bud cells: the degenerative changes resemble early changes in denervated taste buds.
Our observations show that the pattern of degeneration is quite diffierent in the three cell types of the taste buds. The earliest changes in type II and III cells are an increase in cytoplasmic filaments and extreme dilatation of rough endoplasmic cisternae. Profiles of dead type I and II cells vastly increase in number. Type I cells appear less affected by the drug and the only sign of degeneration was an increase in the number of autophagic vacuoles in their cytoplasm.
There was no indication that degenerating type I cells undergo transformation to type II or III cells. These findings support our hypothesis that type I, II and III cells represent distinct cell types and do not undergo transformation to other types in the course of their life span.

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1. In 125 cases of perirenal fat samples derived from human necropsies (from 1 month to 86 years), the occurrence of the brown adipose tissue was examined. Brown fat cells were contained in 72% (90: 125) of the cases.
2. In the infants, all samples contained maximal amounts of brown fat cells: the perirenal fat was composed almost exclusively of brown fat cells: in children and teenagers they began to diminish, and in younger adults further reductions were observed: after the fifth decade conspicuous diminutions occurred. In later decades the brown fat in the perirenal fat was small in amount.
3. This change in the amount of the brown fat tissue undergoes individual variations. In some cases, the brown fat tissue disappeared from perirenal fats in the early stages of life, while in others it persisted to very late stages of life. A man as old as 86 years possessed brown adipose cells in the perirenal fat.
4. Multilocular brown fat cells were classified into the following types: Type 1, fat-depleted cells: Type 2, small-locular cells: Type 3, middle-locular cells: Type 4, large-locular cells: Type 5, monolocular brown fat cells with a thick cytoplasmic rim and pseudomonolocular brown fat cells and Type 6, multilocular brown fat cells rich in cytoplasm. In the infants, all cell types were identified: the small-locular cells were in general scanty in all decades following infancy: in later decades of life, the most common cell types were middle-locular and large-locular cells. The fat-depleted cell is a particular cell type and may occur usually accompanied by multilocular brown fat cells rich in cytoplasm.
5. The fat lobules are composed of a centrally located brown fat cell area and a peripheral layer of monolocular white fat cells, which in the infants was very thin but in the following ages was gradually widened to invade the interior of the lobules.
6. The present findings suggest a continuous replacement of brown fat cells by white fat cells during advancing age. The monolocular brown fat cell with a thick cytoplasmic rim and the pseudomonolocular brown fat cells probably represent transitional forms between multilocular brown fat and monolocular white fat cells. These cell types were found throughout life, suggesting continuous transformation of the brown fat cell into the white fat cell.

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The surface structure of the endocardial endothelium of normal monkeys (Macaca fuscata and M. irus) was studied using scanning and transmission electron microscopy. The endocardium was covered by a layer of endothelial cells, each of which was recognized by the presence of nuclear bulge and marginal folds. The free cell surface was covered by a number of microvilli. The size of the endothelial cell and its surface morphology varied considerably in the different portions of the heart. The endothelial cells were packed more densely along the free margin of the valves, especially at the noduli valvularum semilunarum. The microvilli over the cell surface were denser and longer on the ventricular side of the mitral valve and on the aortic valve, where the marginal folds were not “folds” but were formed by an array of numerous microvillous projections. These cytoplasmic projections had a topographical correlation with micro- and macro-pinocytotic vesicles, thus suggesting their role in the interaction with circulating biologically active substances.

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