structure was, in both cases, enhanced by the use of acid fixatives and by the removal of the embedding, since in sections fixed in buffered osmic acid only a finely granular material is revealed in the nucleus. Thus, as mentioned by Sjöstrand with respect to the cytoplasm (163), little can yet be concluded in this area of the cell.
The nuclear membrane has been shown to possess a very interesting double structure in many cell types. In oöcyte nuclei which were simply spread thinly upon grids, the inner membrane was shown to be continuous, while the outer one consisted of fine pores (34). A similar structure exists in Amoeba proteus. In this case, the continuous membrane is the outer membrane, while the inner one is porous (78). Bairati and Lehmann (6) describe the pores as 1,200 A in diameter with 1,500-A spaces between them, while the continuous layer consists of small, densely packed globular particles. The inner membrane is very resistant to mechanical action and to different fixing fluids, and it contains few lipoids. In sections of these cells, Harris and James; (78) describe the outer layer as 1,000 A thick and the inner layer as 2,000 A thick. Their measurement for the space in between pore centers is 1,200 A with a pore diameter of 800 A.
The membranes of nuclei of animal tissue cells have also been shown to be double, although of much finer dimensions than those described above, while no finer structure within the membranes has yet been revealed. The doubleness was first shown in neuronal nuclei by Hartman (79). Such doubleness has now been observed in kidney cells (163) and in all cells under our investigation. Sjöstrand (163) was able to measure the total thickness of a nuclear membrane in the kidney tubular cells as 230 A units with a distance between the centers of the single membranes as 160 A units. This makes the calculated thickness of a single membrane as 60 A units and the height of the space in between 100 A units. Although such accurate measurements have not been made on nuclei of other cell types, they all appear to be of the same order of thickness (157).
It has been the purpose of this article to demonstrate the manner in which electron-microscopic studies of biological tissues may be made, the information which such studies have yielded to date, and their potential contributions in future years. It was necessary to spend most of the first decade of the availability of the electron microscope in developing methods to prepare biological specimens sufficiently dry, suffi