1. Towards the basement membrane the cell membrane appears as a thin zone of condensed cytoplasm. An about 200 Å wide space separates this cytoplasmic zone from the basement membrane. Between adjacent cells the separating membrane consists of an about 160 Å thick double membrane, which connects to the “brush border” structure.

2. The cytoplasm of the basal cell zone is divided into compartments by means of double membranes here called intracellular cytoplasmic membranes. They surround the mitochondria and at the basal cell surface they assist in lining the cytoplasm towards the basement membrane. The mean thickness of these membranes is 270 Å, the thickness of each constituent single membrane is 80 Å and the height of the space in between the two membranes is 110 Å.

3. The mitochondria are rod-shaped. They are lined by a 160 Å thick double membrane consisting of two about 45 Å thick membranes separated by a 70 Å wide space. In the interior of the mitochondria there are transversally oriented double membranes organized in a similar way as the outer mitochondria membrane and mutually separated by 170–470 Å wide spaces. Between these internal membranes areas exhibiting a pronounced contrast due to a heavy reduction of osmium tetroxide may be found.

4. The “brush border” consists of densely arranged cylindrical “ducts.” The “ducts” are 600 Å wide and 1.0–1.5 μ long and lined by a thin dense wall. These ducts are closed towards the tubular lumen. They are also closed at their base where the walls continue in curved rods. The “ducts” are oriented perpendicularly to the cell surface facing the lumen and in cross sections they closely resemble the structure of a honey-comb with the exception that the tubes approach a circular, more than a hexagonal cross section.

5. In the intermediate cell zone there are granules and vacuoles, corresponding to the previously described fluorescent granules (18). These structures exhibit a well marked membrane surrounding the vacuole or the granule. The granules contain inner granules which are extremely dense due to heavy reduction of osmium tetroxide. The granules of this cell zone correspond to the osmium tetroxide reducing granules observed in the light microscope and which have been classified as constituting the Golgi apparatus of these cells.

6. The nuclei are lined by a 230 Å double membrane, the constituent single membranes measuring 60 Å in thickness and the height of the space in between these single membranes 100 Å.

7. The structure of the characteristic double membranes observed as cell membranes, intracellular cytoplasmic membranes, outer and inner mitochondria membranes and nuclear membranes is discussed and it is tentatively assumed that they are composed of two protein membranes separated by multilayers of lipid molecules.