The echiuroid marine worm Urechis caupo is uniquely suited for the study of oogenesis. A relatively large quantity of oocytes at various developmental stages can be obtained and subjected to coordinated cytochemical and biochemical analysis Oocytes at the cluster, early diplotene, mid-diplotene, and diffuse diplotene or lampbrush stages are active in the synthesis and accumulation of ribosomal RNA, several proteins, carbohydrates, lipids, and also, perhaps, yolk constituents. Only cortical granule formation, which occurs during later stages of oogenesis, appears to be stage specific. Ribosomal RNA genes are also transcribed in the nucleolus of the mature oocytes or unfertilized eggs. However, the rate of production in these eggs appears to be regulated at the level of maturation of rRNA precursor molecules.
