BMP-4 and activin, or BMP-4 and FGF treatment of animal caps induce more of o-dianisidine–positive cells than BMP-4 treatment alone. The animal caps were treated as described in Fig 3. The o-dianisidine–positive cells were counted from two cytospins for each experimental condition in a representative experiment. As each cytospin represents three caps, the sum ofo-dianisidine–positive cells from two cytospins was divided by six to give the number of erythroid cells per cap.