Fig. 7. A model for patterning of the vegetal hemisphere in ascidian embryos. (A) Patterning in normal embryos. Cell types are highlighted by the same color code as in Fig. 1. At the division to the 64-cell stage, two kinds of asymmetric division take place in the anterior (NC vs. Not) and posterior (Mes vs. Mus) marginal zones. macho-1 is a maternal and intrinsic competence factor for mesenchyme induction. Light blue arrows indicate FGF signal. (B) Direction of asymmetry for segregation of muscle and mesenchyme fates is determined by the direction from which the FGF signal comes. (C) Polarity of asymmetric division that produces nerve cord and notochord precursors is determined by the direction from which the inhibitory signal to suppress notochord fate comes. The signal inhibits activation of MAPK by the FGF signal and expression of FoxA on the nerve cord side. Presence of the ectodermal signal seems to be more crucial for generation of the medial nerve cord precursors (A7.4 cells in Fig. 1) than that of the lateral precursors (A7.8 cells). See text for details. Ecto, ectoderm; En, endoderm; Mes, mesenchyme; Mus, muscle; NC, nerve cord; Not, notochord.