The principal structural compartments of the macronucleus of Euplotes eurystomus were examined by ultrastructural and cytochemical procedures. Interphase chromatin is condensed in highly compact granules that stain intensely with the DNA-specific osmium-amine procedure. Nucleoli react strongly with silver and with thiol-specific reagents, but are almost completely unstained by osmium-amine. The organelle of DNA synthesis, the replication band, is composed of 2 zones. The forward zone consists of highly ordered chromatin fibers, stains strongly with osmium-amine, with silver, and with thiol-specific reagents. The rear zone, which is the site of DNA synthesis, is impoverished in DNA, and is very sensitive to collapse induced by in vivo heat shock, or during nuclear isolation.
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