On the basis of light, autoradiographic (uridine-3H incorporation) and electron microscopic investigation changes of nuclear structures were examined during the oogenesis in Chrysopa perla L. — In early meiotic prophase the oocyte nuclei were found to contain a large body of extrachromosomal DNA. In certain cases the latter splits up into several DNA clumps giving rise to a few (4–7) primary nucleoli, 3–5 μ in diameter. The primary nucleoli consist of densely packed fibrils 50–100 A thick. They contain no granular component and are inactive in RNA synthesis. — At the beginning of large growth the extrachromosomal DNA bodies disappear and numerous electron-dense clumps, 0,5–1 μ in diameter, appear in the nucleus. Instead of the primary nucleoli, the nucleus now contains a great number of ring nucleoli about 0,5–1 μ in diameter with a granular component (granules are 150 A). The space between them is filled up with nucleolar strands running from the surface of the ring nucleoli. — At the stage ring nucleoli of uridine−3 H incorporation into the oocyte nucleus begins. — During later previtellogenesis and at the beginning of vitellogenesis the ring nucleoli disappear and the nucleus is filled with the network of nucleolar strands. Among them there are specific complexes. These consist of electron dense masses, of granular clusters (granules 500 A in diameter) and large fibrillar electron light bodies. At this stage the nucleus takes the most active part in RNA synthesis. — The process of karyosphere capsule formation was studied by electron microscopy. The capsule was found to be of fibrillar nature; its structure is very peculiar and unlike any known membrane components of the cell. On the basis of cytochemical evidences the characteristics of the capsule are given. — The development of a powerful nucleolar apparatus based on the extrachromosomal DNA and a possible role of the synaptonemal complex and extrachromosomal DNA in formation of the karyosphere capsule is discussed.