Test cell differentiation during oogenesis and early embryogenesis of Botryllus schlosseri (Ascidiacea).

Appearance and differentiating of test cells were followed in the ascidian Botryllus schlosseri, using colonies grown in the laboratory. Presumptive test cells were first recognizable during late pre-vitellogenesis as isolated cells sited under the primary follicolar layer. Then they embedded themselves in depressions of the oocyte being partially covered by its microvillar extrusions. Junctions definitely identified as such were neverseen to be established between test cells and the oocyte. The main feature of differentiation consisted in the development of endoplasmic reticulum and Golgi apparatus composed of numerous long cisternae, and in the formation of dense round granules. The latter underwent morphological changes at the end of oogenesis and during early embryogenesis, when the test cells were freed into the perivitelline space. These observations suggest that the role of test cells in B. schlosseri is mainly played during these stages.