Since the Urechis unicinctus-oocyte grows asynchronously in the body fluid, various oocytes in developmental stages can be prepared from each individual. The oocytes obtained from the coelomic fluid are then classified into five developmental stages according to the fine structural features. The earlier oocytes (${sim}18{mu}m$) form cluster and thereafter the oocytes grow singly without a distinct support of somatic cell, such as accessory cell or matrix cell. The yolk granules begin to appear already in the oocyte of cluster stage, however, the typical yolk was observed at the stage IV. Therefore, it was suggested that the yolk deposition is correlated with the coelomic fluid. The mature oocyte measured about $150{mu}m$ produces the invagination not only on oolemma(indentation) but also on nuclear envelope. After the formation of the indentation, the mature ooytes are stored in storge sacs. The fine structural features were combined in aspect of structural concept of light microscopical observation.