Erythroid cells are detected in animal caps treated with BMP-4 and mesoderm inducers. (A) For experiments using plasmid BMP-4 embryos are injected with control vector or pcDNA3–BMP-4 (200 pg) and treated with activin (12 ng/mL) or FGF (200 ng/mL). (B) For experiments using BMP-4 protein, animal caps are excised at stage 8 and dispersed in CMFM. The cell dispersion is treated with BMP-4 (1 μg/mL) with and without activin (12 ng/mL) in CMFM for 2 hours before the cells are washed and allowed to reaggregate in the calcium and magnesium containing cap culturing solution. In both experiments the cap cells are cultured until control stage 35 and analyzed foro-dianisidine–positive cells. The arrowhead points to an erythroid cell. The black arrow indicates a nonerythroid pigmented cell. The panels in (A) were photographed at 20× original magnification and the panels in (B) were photographed at 10× original magnification.