Changes in the distribution of circulating red cell populations between E10.5 and E17.5 of mouse gestation. (A) Changes in the percentage of nucleated (white columns) and enucleated (black columns) βH1-globin–positive primitive red cells in the bloodstream. There is a progressive transition from nucleated to enucleated forms. (B) Changes in the distribution of βH1-globin–positive nucleated (white columns) and enucleated (black columns) cells and in βH1-globin–negative nucleated (clear columns) and enucleated (gray columns) cells. Primitive red cells become a progressively smaller component of the circulation between E12.5 and E17.5. Results in this figure are derived from the examination of 14 855 cells from more than 60 fetuses from 22 timed litters.