Blocking BMP signaling during neurulation reduces differentiation markers of primitive hematopoiesis. Northern blotting analysis using 0.8 μg per lane of polyA⁺ mRNA harvested from uninjected embryos or embryos injected at the 2-cell stage with 2 ng/embryo of ER-xSmad6 mRNA and induced with E2 at the stages indicated (NH = no hormone). mRNA was harvested from stage 22 embryos (A) to test early markers of primitive hematopoiesis (GATA-2 and SCL) or from stage 35/36 embryos (B) to test late markers (GATA-1A, neptune, and αT₁-globin). Numbers above lanes indicate percent of signal relative to the sample that is uninduced (NH).