Fig. 8. A directed signal and asymmetric division model of tissue specification mechanism in the vegetal hemisphere of the ascidian embryo. The model is applicable to both the anterior and posterior margins of the vegetal hemisphere. (A) Schematic drawing representing embryo at 32-cell stage. Endoderm precursors (En) emanate inductive FGF-like signal (green arrows) to neighboring anterior and posterior blastomeres and polarize them. Posterior-vegetal cytoplasm (PVC; red oblique lines) causes different responsiveness in posterior marginal cells. (B) Asymmetric divisions occur at the 64-cell stage. In the anterior region, one daughter cell that faces the inducer and does not have the PVC assumes notochord fate (Not). In the posterior region, one daughter cell that faces the inducer and contains the PVC adopts mesenchyme fate (Mes). (C) Without inductive signal, both daughter blastomeres in the anterior region assume default nerve cord fate (NC), and those in the posterior region assume default muscle fate (Mus). (D) When isolated blastomeres receive FGF signal all over the surface, both daughter cells develop into notochord or mesenchyme, depending on absence or presence of PVC.