Development of primitive erythroid cells (EryPs) in the mouse. (Top) EryPs do enucleate but the whole process from onset of nuclear condensation at E10.5 to completed extrusion of nuclei at E15.5 is much slower than for definitive erythroid cells (EryDs; 2 days). At E14.5, approximately equal numbers of EryPs and EryDs are observed in embryonal blood. Data taken from Fraser et al.⁵  (Bottom) Entry of earliest EryPs into circulation is accompanied by a wave of gene activation. A second distinct wave of up- as well as down-regulation occurs during maturation in the fetal liver.¹  Cell and nuclear diameters to scale.