xSmad1, but not xSmad2, can rescue the ER-xSmad6–induced block of primitive erythropoiesis. Embryos were coinjected at the 4-cell stage in the ventral blastomeres with 1 ng/embryo ER-xSmad6 mRNA and either 0.6 ng/embryo of EF-1α (A,D), xSmad1 (B,E), or xSmad2 (C,F) mRNA. Half of each cohort of injected embryos was cultured in the presence of E2 from stage 10.5 (D-F). All embryos were fixed and stained with benzidine at stage 35. Original magnification, × 20.