Oogenesis and oocyte envelope differentiation in the viviparous ascidian Botrylloides violaceus.

Oogenesis and oocyte envelope differentiation were examined in the viviparous Botrylloides violaceus, a compound ascidian producing small alecithal eggs about 80 \u3bcm in diameter. Oogenesis was divided into five stages, defined according to ultrastructural changes in envelopes and oocyte growth. In the gonadal blastema of young buds, each oocyte in the early meiotic prophase is partially enveloped by a few primary follicle cells (stage I) which adhere to the oolemma at several junctional points. In stages II and III, these cells give rise to an external continuous layer (presumptive outer follicle cells) and to underlying scattered elements which later differentiate into test cells and inner follicle cells (stage IV). The direct contribution of mesenchymatous elements was not observed in inner follicle cells and test cells. Observations on the time of appearance and development of organelles, microvilli and endocytotic activity in growing oocytes are reported. Oocytes progressively accumulate \u201cclear\u201d vesicles containing a loose mat and small spherical bodies resembling the previtellogenic bodies observed in other botryllids; they are seen to receive contributions from Golgian vesicles. However, the typical electron-dense yolk globules described in the oocytes of ascidians so far examined were never observed. The meaning of these clear vesicles and the possible role of envelopes in oocyte development are discussed, and the small alecithal eggs of B. violaceus are compared with the yolky eggs of other ascidians