In the foetal erythropoiesis in mouse foetuses, a change from nucleated erythroblasts to anuclear erythrocytes takes place within a period of a little more than 24 hours. This change is due to extrusion of the nuclei. The present findings give rise to the assumption that it would be reasonable to consider the primitive erythroblasts which are characteristic of early foetal erythropoiesis as well as the subsequent definitive erythroblasts to be links in a continued and gradual developmental series, not as two generations of foetal blood cells. The possibility of nuclear extrusion in human foetal erythropoiesis and medullary erythropoiesis is discussed.