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Tetrahymena in the log phase of growth were pulse labeled with uridine-3 H, fixed in aceticalcohol, extracted with DNase, and embedded in Epon . 0 .5-µ sections were cut, coated with Kodak NTB-2 emulsion, and developed after suitable exposures. Grains were counted above macronuclei, above 1000 micronuclei, and above 1000 micronucleus-sized "blanks" which were situated next to micronuclei in the visual field by means of a camera lucida . An analysis of grain counts showed that micronuclei were less than ?200o as active as macronuclei on the basis of grains per nucleus . Since micronuclei contained, on the average, about ?s0 as much DNA as macronuclei, micronuclear DNA had less than 1 o of the specific activity of macronuclear DNA in RNA synthesis. However, even this small amount of apparent incorporation was not significantly different from zero . Comparisons of the frequency distributions of labeled micronuclei with those of micronuclear "blanks" showed no evidence of a small population of labeled nuclei such as might be expected if micronuclei synthesized RNA for only a brief portion of the cell cycle . We conclude from these studies that there is no detectable RNA synthesis in Tetrahymena micronuclei during vegetative growth and reproduction .

A B S T R A C TIn order to investigate some of the cytochemical processes involved in interphase growth and culminating in cell division, a combined autoradiographic and microphotometric study of nucleic acids and proteins was undertaken on statistically seriated cells of Vicia faba root meristems. Adenine-8-C x4 and uridine-H ~ were used as ribonucleic acid (RNA) precursors, thymidine-H 3 as a deoxyribonucleic acid (DNA) precursor, and phenylalanine-3-C 14 as a protein precursor. Stains used in microphotometry were Feulgen (DNA), azure B (RNA), pH 2.0 fast green (total protein), and pH 8.1 fast green (histone). The autoradiographic data (representing rate of incorporation per organelle) and the microphotometric data (representing changes in amounts of the various components) indicate that the mitotic cycle may be divided into several metabolic phases, three predominantly anabolic (net increase), and a fourth phase predominantly catabolic (net decrease). The anabolic periods are: 1. Telophase to post-telophase during which there are high rates of accumulation of cytoplasmic and nucleolar RNA and nucleolar and chromosomal total protein. 2. Post-telophase to preprophase characterized by histone synthesis and a diphasic synthesis of DNA with the peak of synthesis at mid-interphase and a minor peak just preceding prophase. The minor peak is coincident with a relatively localized DNA synthesis in several chromosomal regions. This period is also characterized by minimal accumulations of cytoplasmic RNA and chromosomal and nucleolar total protein and RNA. 3. Preprophase to prophase in which there are again high rates of accumulation of cytoplasmic RNA, and nucleolar and chromosomal total protein and RNA. The catabolic phase is: 4. The mitotic division during which there are marked losses of cytoplasmic RNA and chromosomal and nucleolar total protein and RNA.A primary area of cytology concerns the complex and periodic physiological changes that accompany the mitotic cycle. It has become increasingly evident that much of the mitotic process involves a utilization and segregation of materials synthesized during the preceding interphase. It would seem, therefore, that a study of the time and rate of synthesis in the mitotic cycle of the major nucleoprotein fractions of the cell, would form one approach to an understanding of mitosis and interphase growth. In the present study, the nucleoproteins of the nucleolus, chromosomes, and cytoplasm have been studied using both microphotometric and autoradiographic methods. A study of this kind may be made on a homogeneous tissue, the cells of which are undergoing a division cycle which is synchronous, or if non-synchronous, cells may be related to time in one of a number of ways: seriated by selection of cells at the same stage of mitosis (Prescott, 1955) synchronized by tempera-

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