Figure 2:

Conditional specification of endomesoderm in the sea urchin. At the 32-cell stage (A) the large micromeres (purple) first activate delta expression. The neighboring macromeres (magenta) display a high level of nuclear β-catenin that, along with the Delta input, initiates a conditionally specified endomesoderm. These signals persist through at least the 8^th cleavage. (C) Eighth cleavage divides Veg2 cells into two tiers. The anterior progeny (the Veg2 upper tier) lose contact with the micromeres and therefore with the Delta signal thereby losing their mesodermal potential, while the Veg2 lower cells remain in contact with the Delta producing micromeres and adopt a mesodermal fate. (D) At the beginning of gastrulation, the micromeres undergo an epithelial-mesenchymal transition to become the PMCs, and the non-skeletal mesoderm will soon initiate invagination of the archenteron, followed by invagination of the Veg2 endoderm and finally the Veg1 endoderm.