Nucleated circulating red blood cells (RBCs) of developing zebrafish, chick and mouse embryos can actively proliferate. While marrow- or organ-mediated erythropoiesis has been widely studied, transforming processes of circulating RBCs are under little scrutiny. We employed confocal, stereo- and electron microscopy to document the maturation of intravascular RBCs. In zebrafish embryos (32-72 hours post fertilization), in the caudal vein plexus follows a four-step program: (i) Nuclear division with continued cytoplasmic connection between somata. (ii) Dumbbell-shaped RBCs tangle at transluminal vascular pillars. (iii) Elongation, and (iv) Disruption of soma-to-soma connection. Dividing RBCs of chick embryos, however, retain the nucleus in one of their somata. Here, acts to pinch-off portions of cytoplasm, organelles and ribosomes. Dumbbell-shaped primitive RBCs re-appeared as circulation constituents in mouse embryos. The of circulating RBCs, thus, represents a biologically relevant mechanism of RBC division and maturation during early vertebrate ontogeny.