The nuclei of two spherical bacteria have been examined in electron micrographs of thin sections of specimens prepared by the method of Ryter and Kellenberger (1958). The nuclei appear to consist of the same fine fibers in a matrix of low density which have already been seen in many other bacteria prepared by the same procedure. They are worth a separate description because their constituent fibers are arranged in patterns of uncommon orderliness. In the nuclei of one of the two bacteria this is seen at all times, in the nuclei of the other one only at the beginning of the growth cycle. In some places the diameter of the nuclear fibers is close to that of the DNA molecule in the model of Watson and Crick (1953).

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