The extent to which bone transplants survive and the source of any new bone subsequently formed are still matters of controversy. Three main hypotheses have been put forward.

During the early stages of the establishment of a 'pure' strain of endosteal fibroblasts in vitro, it was observed that a number of actively motile giant cells of unusual type were appearing fairly regularly among the cells growing out from explanted pieces of embryonic bone. As no description of similar cells in vitro could be found in the literature it was decided to investigate their properties, and, if possible, their source. MATERIALS AND METHODSThe usual routine for hanging-drop tissue culture was employed; small fragments of membrane -bone, obtained from the frontal bone of 13-day chick embryos, were explanted on to coverslips in a cock-plasma clot. After incubation for the requisite period, the cultures were fixed in Carnoy's fluid and stained subsequently either with carmalum, Meyer's haematoxylin and eosin, or Heidenhein's iron haematoxylin. The cultures were mounted in toto in balsam. RESULTSExamined after 24 hr. incubation, the cultures showed the usual outgrowth of an actively proliferating inner zone of spindle-shaped fibroblasts, and an outer zone comprising wandering cells which were both microphage and macrophage in type (P1. 1, fig.

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Timeqapse motion picture studies were carried out on isolated fowl embryo osteoclasts in vitro; the cells have an extremely active ruffled border, and show vigorous pinocytotic activity. Electron microscope studies on osmium-fixed cells showed that the pinocytotic vacuoles contained bone salt crystals (as well as material which could not be identified on morphological grounds), and that the folds of the ruffled border enclosed crystals and collagen fibrils. Changes were seen in the matrix beneath the ruffled border. Initially, the collagen fibres became separated from each other and at the same time bone salt crystals became detached from them. Later, as crystals and ground substance disappeared, the outline and cross-striation of the collagen became distinct. The implications of these findings are discussed with respect to the mechanism of bone erosion.

There is evidence that the renal collecting tubule is involved in the final concentration of urine, but the fact that its wall is built up of two different kinds of cells has not hitherto been taken into account. The possible r6le of one of these, the "dark" cell, has been investigated by quantitative and qualitative methods. Collecting tubules were isolated by digestion in collagenase from the kidneys of normal rats and of rats deprived of water for 48 hr.; the dark cells were identified by staining with the Nitro BT method for succinic dehydrogenase. The number of dark cells was counted; the tubules were classified on a percentage basis into three groups, containing less than 30, 30-50, and more than 50 dark cells respectively; cytological and cytochemical observations were also made. It was found that both quantitative and qualitative changes occurred in the dark cells of the dehydrated rats. It seems that these cells secrete into the tubule lumen. It is suggested that collecting tubules may differ from each other functionally. The results are discussed in relation to Ginetzynski's hypothesis that ADH causes hyaluronidase to be secreted into the collecting tubule.THE view that the collecting tubule takes an active part in the concentration of urine rests upon two kinds of evidence. There is first, the measurement of osmotic pressures of tubular fluids obtained by micropuncture methods as reported by Wirz [1957] and Gottschalk and Mylle [1959]; and second, the histological and histochemical approach as typified by the work of Ginetsynski. In 1954 he reported that urine contains hyaluronidase and that more is present during osmotic diuresis. Later Ginetsynski [1958] described histological and histochemical investigations on the kidneys of rats deprived of water or injected with antidiuretic hormone (ADH); the mucopolysaccharides in the basal membranes of the collecting tubules became depolymerized and surrounding lymphatics were dilated. He concluded that ADH acts by stimulating the collecting tubule epithelium to produce hyaluronidase which depolymerizes polysaccharides in the tubule walls and surrounding connective tissue. They become permeable to water, which leaves the tubule and enters the lymphatics.