Unfertilized eggs of the ascidian Halocynthia roretzi are radially symmetrical along the animal-vegetal axis. After fertilization, ooplasmic segregation results in formation

In the unfertilized eggs of many kinds of animal, there is a single axis, namely, the animal-vegetal axis, and eggs exhibit radial symmetry along the axis. The second axis is established just after fertilization or during early embryogenesis, and embryos become bilaterally symmetrical. In the ascidian egg, the second axis, which corresponds to the future anteriorposterior axis, is generated during ooplasmic segregation. Movements of the ooplasm progress in two phases between fertilization and the first cleavage (Conklin, 1905; Hirai, 1941; Sawada and Osanai, 1981; Jeffery and Meier, 1983; Sardet et al., 1989). During the second phase of ooplasmic segregation, vegetally located cytoplasm moves towards the future posterior pole (Fig. 1). This movement is the first observable evidence of an anterior-posterior axis during ascidian embryogenesis. The study of ascidian embryogenesis has provided various types of evidence for the presence of localized maternal factors, in specific regions of the egg cytoplasm, that play important roles in the determination of developmental fates during early embryogenesis (reviewed by Venuti and Jeffery, 1986; Meedel, 1992; Nishida, 1992b; Satoh, 1993). Indeed, experiments involving redistribution of cytoplasm and transfer of cytoplasm have revealed the presence of cytoplasmic determinants that confer on muscle-, endodermand epidermislineage cells the ability to become muscle, endoderm, and epidermis, respectively (Whittaker, 1980, 1982; Deno and Satoh, 1984; Nishida, 1992a, 1993, 1994). These three kinds of cytoplasmic determinant move in different directions during ooplasmic segregation. Prior to the onset of the first cleavage, three kinds of determinant settle at sites corresponding to specific regions of the future bilateral fate map (Nishida, 1994). In addition to determinants of cell type, cytoplasmic determinants for gastrulation movements are also present in the ooplasm. Ortolani (1958) bisected fertilized eggs near the equator and showed that the animal fragments developed into permanent blastulae. Bates and Jeffery (1987) showed that when small region of vegetal-pole cytoplasm was removed from the zygote between the first and second phase of segregation, the embryos fail to gastrulate. The removal of cytoplasm at the vegetal pole could be mimicked by UV irradiation of the fertilized egg near the vegetal pole (Jeffery, 3093 Development 120, 3093-3104 (1994) Printed in Great Britain © The Company of Biologists Limited 1994