Microscopic examination of the pigment cell aggregations in the liver and spleen of mud turtles (Kinosternon flavescens) suggests that the morphology and behavior of these cells is consistent with the melanomacrophages described in teleost fishes and the pigmented "Kupffer cells" described in frogs and reptiles. These cells contain massive amounts of melanin, substantial lipofuscin, and some hemosiderin consistent with their phagocytic function. Similar-appearing isolated pigmented macrophages are solitary in the liver, spleen, lung, and kidney. Number and size of the largest hepatic aggregations increase almost linearly with turtle age so that in old turtles they may constitute up to 20% of the liver volume. This increase may result from hepatic recruitment of macrophages throughout the life of the turtle and suggests that size and number of melanomacrophage aggregations may serve as a marker for senescence in otherwise healthy turtles of this species.

This work was carried out with the intent of developing a method capable of routinely evaluating calf thymus nuclear preparations with the electron microscope. Examination of small random samples, pre-embedded in agar after fixation with permanganate, were found to give results comparable to those obtained with much larger samples withdrawn randomly from pellets and embedded and sectioned conventionally. Results obtained by this pre-embedding technique with acrolein, osmium tetroxide, or permanganate fixations were equivalent. Calf thymus nuclear preparations isolated in sucrose by the method prevalently used (see 1) are contaminated only slightly with intact cells, to a degree which varies with each preparation. However, intact cells, damaged cells, or nuclei with some cytoplasm constitute together about 30 per cent of the preparation. Particles other than intact cells are not readily distinguishable from one another by light or phase microscope techniques. These preparations can be purified further by centrifuging through a dense sucrose layer. In our hands, however, contamination with some cytoplasm still remains in approximately 10 per cent of the particles. Incubation of the particles prepared without purification procedures, under conditions frequently used, results in the extensive breakdown of particles. Under at least one set of conditions, nuclei are selectively disrupted, leaving primarily damaged cells in the preparation.

A study of the permeability of calf thymus nuclei isolated in sucrose was carried out with sucrose-1 4 C, glycerol-14C, and carboxydextran-14C (molecular weight, 60,000-90,000). The results indicate that the nuclei are very permeable to both sucrose and glycerol but they exclude the carboxydextran. Results obtained with other low molecular weight nonelectrolytes (malonamide-1 4 C, erythritol-14C, D-arabinose-4 C, and D-mannitol-4 C) are in agreement with the view that the nuclei are freely permeable to these molecular species. A sucrose-impermeable space is also present in these preparations and it has been attributed to the presence of intact cells. The high permeability of nuclei to sucrose was confirmed with Ficoll-separated preparations. The possibility of the presence of a substantial particulate space that allows the penetration of dextran cannot be excluded by these experiments, and this space may correspond to damaged nuclei.