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Abstract The central nervous system of rabbit fetuses from the ninth to the twentieth day of gestation has been examined by phase contrast and electron microscopy. Blood vessels are first seen in the neuroectodermal tissue at 11 days of gestation. They increase in number between 11 days and 20 days of gestation but there is no change in their fine structure. The endothelial cells usually have voluminous cytoplasm which forms projections into the lumen of the vessels. These projections or flaps are a prominent feature of the fine structure of the endothelial cells during this period of growth of the central nervous system. Their function appears to be the impounding of material from the blood stream by curving over and coalescing with the plasma membrane, thus forming a large vesicle. Thorium dioxide, injected into the blood stream, is taken up along with the constituents of the plasma and is seen in the large vesicles formed by the flaps, as well as in smaller vesicles formed directly from the luminal plasma membrane by the usual mechanism of pinocytosis. Flaps are usually located at the terminal bars. Thorium dioxide is not seen between the thickened cell membranes that form the terminal bar but is present in vesicles adjacent to the terminal bar.
Sheila A. Donahue (Wed,) studied this question.