Comparison of morphology of erythroid cells induced in animal caps to those found in tadpoles and adult frogs. (A) An erythroid cell from an animal cap treated with BMP-4 and FGF; (B) circulating erythroid cells in a 3-day-old tadpole (stage 41); (C) adult erythroid cells. The erythroid cells are stained with Wright Giemsa. The primitive erythroid cells in (B) are 1 day older than and look slightly more differentiated than the cell shown in (A). Primitive erythroid cells are larger and more circular than definitive erythroid cells; they contain circular and less condensed nuclei and still possess yolk granules (compare B with C). The panels are photographed at 40× original magnification.