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The cartoon depicts the emergence of primitive erythroid progenitors in the blood islands of the yolk sac beginning at embryonic days (E) 7.5-8.5 in the mouse. A primitive erythroid lineage is found in all vertebrate species and is the first mesodermal cell type specified at the end of gastrulation. In the blood islands, clusters of erythroid progenitors are surrounded by endothelial cells. Progenitor activity decreases and is lost just around the initiation of circulation (~ E9.0). As the heart begins to beat, progenitors enter the bloodstream and continue to mature as a nearly synchronous cohort, condensing and eventually extruding their nuclei (E12.5-E14.5). The stepwise maturation of these cells is grossly reflected in their nuclear size and Giemsa-staining properties, depicted using different colors. See the article by Isern et al on page 4924.