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Purified human globin injected into rats forms aggregates which are identifiable by their characteristic appearance in thin sections in the electron microscope and by their positive autoradiographs when the globin is tritiated before injection. Globin is taken up by endothelial cells of glomerular capillaries and is transported across the cell within the limits of a surrounding membrane. Globin is rarely seen to pass through fenestrations. Globin is also taken into the stalk region where it is seen usually within the sponge fibers and only occasionally within stalk cells. Globin is seen in all stages of passage through the basement membranes and sponge fibers, which are not deformed by its passage. On the basis of the findings presented here and by others, it is postulated that the basement membrane and sponge fibers consist of a thixotrophic gel. After traversing the basement membrane, the globin passes between foot processes of the epithelial cells. The slit membranes are deformed by this passage and thus appear to be distinctive structures. The globin is next found free in Bowman's space; the earliest aggregates are seen there within 1 minute after injection. Globin taken up in the stalk region is slowly discharged and very little is found there 6 hours postinjection.

The light microscopy and ultrastructure of a malignant epithelioid schwannoma are described. Characteristic cells resembling perineural elements with various degrees of differentiation were observed. Primitive epithelioid cells contained scant ergastoplasm, and few tubules and filaments, but did have abundant free ribosomes and Golgi membranes. Also noted were junctional complexes and focal fusion of plasma membranes, basal laminae were absent. Better differentiated cells were completely limited by a well-developed basal membrane and had an abundance of intracytoplasmic filaments and multiple pinocytotic vesicles. The intercellular ground substance was composed of numerous fine collagen fibrils and amorphous, basement membrane-like, electron-dense material. A striking ultrastructural similarity of the tumor cells to those encountered in ethylnitrosourea-induced malignant schwannomas in rats was noteworthy.

The median eminence of the rabbit has been studied electron microscopically and the neurosecretory granules in the nerve terminals of the external layer were shown to be smaller and have a denser central core than the granules composing Herring bodies which are generally larger and have a paler core which is finely granular or vesicular. It was proposed that the small dense granules, which may represent neurosecretory substances destined to reach the pars distalis via portal veins, reach the nerve terminals in the external layer via certain dilated fibers described in this paper which contain identical granules. Some may also arise from the small dense granules occasionally seen in Herring bodies.The demonstration of neurotubules within Herring bodies and the continuity of nerve fibers into Herring bodies, as well as the demonstration of myelin around some aggregates of large pale granules such as those seen in Herring bodies are further evidence that Herring bodies are dilated axons. Wide perivascular connective tissue spaces and interfibrillary spaces were generally seen in the external but not the internal layers. The absence of neurosecretory granules from the perivascular connective tissue spaces suggests that neurosecretory granules go into solution before entering the connective tissue space. Some fibers containing neurosecretory granules, however, penetrated through the basement membrane into the perivascular spaces and certain unidentified electron dense granules were seen beneath the basement membrane of blood vessels.