ULTRASTRUCTURAL STUDIES ON THE NEMATODE XIPHINEMA-DIVERSICAUDATUM - OOGENESIS AND FERTILIZATION

Oogonia in the germinative zone of the ovary are irregularly shaped and lie adjacent to each other or separated by processes of the epithelial cells of the ovary. Developing oocytes pass in single file up to the growth zone and fibrogranular formation occurs around their nucleus. The perinuclear deposits remain until the oocyte is fully grown. Oocytes increase rapidly in volume because of the production of secretory granules. Three types of granules are recognizable. Type 1 granules are spherical, amorphous in structure and delimited by a fighter area, probably consisting of lipoprotein. Type 2 granules, electron lucent, arranged in groups, are lipid inclusions. Type 3 are dense spheres and may represent yolk bodies. The two last are then utilized by the developing embryo. Mature oocytes assume a smooth, cylindrical configuration as they traverse the oviduct. A cone of fertilization seems to be formed at the distal pole of the oocyte, where the sperm penetrates. The sperm totally penetrates the oocyte, through an invagination formed at the oocyte surface. The oocyte continues to undergo two unequal cytoplasmic divisions, resulting in the formation of a female pronucleus and two polar bodies. Under the stimulus of fertilization, a new egg cell membrane is produced, the first one becoming the vitelline envelope.