Electron Microscopy of the Macronucleus of Euplotes Eurystomus

that the macronucleus of Euplotes is excellent material for studying relations between DNA, RNA, and protein synthesis during nuclear replication. Replication bands normally originate at the two tips of the long, rod-shaped macro-nucleus during the early hours of interphase and progress as two waves of D N A synthesis until they meet in the center. In the light microscope one sees that two marked changes in the cytological organization of D N A precede D N A replication. T h e reorganization or replication bands consist of two zones. At the forward edge of the forward zone D N A shifts from a granular state to an evenly dispersed state. In the rear zone, where D N A and histone syntheses occur, DNA is much less concentrated. In the remaining portion of the rear zone, chromatin aggregates are reconstituted ! from the duplicated material. Faur6-Fremiet, Rouiller, and Gauehery (4), a~d Roth (5) have reported observations on the macronucleus and the replication band in Euplotes with the electron microscope. The present study confirms some of the earlier observations and also includes descriptions of several macronuclear structures not previously reported. Particular attention is devoted to the uhrastructure of the FIGURE ] Electron micrograph of a small part of an Euplotes macronucleus stained with 1 per cent lanthanum acetate and 2 per cent uranyl acetate. The small, very dense particles are due to the lanthanum acetate which seems to deposit selectively on the chromatin bodies (C) and not on the nuclear bodies of lower density (N). FIGURE Electron micrograph of a macronucleus treated in the same manner as the one shown in Fig. 1. Rows of particles possibly representing helices are indicated (H). C, chromatin body; iV, nuclear body of lower density. X 54,000 FIGURE 3 Macronucleus stained with 0.1 M ferric chloride. The replication band moves down the macronucleus in the direction indicated by the arrow. The chromatin bodies (C) at the bottom of the figure have already begun to break up and form the network of twisted microfibrils (F). NM, nuclear membrane. SZ, zone of DNA synthesis. X 11,000. FIGURE 4 Macronucleus stained with 0.1 M ferric chloride. The zone of DNA synthesis (SZ) is immediately followed by a region where the minute microfibrils appear to be aggre-gating into small chromatin bodies (C). N, nuclear body of lower density. F, network of twisted microfibrils. X 54,000.