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intermediate form of development of a red blood cell, or erythrocyte. In mammals and in man, erythroblasts have nuclei and reproduce themselves, unlike mature erythrocytes, which lack nuclei and are unable to reproduce.
In the lower vertebrates (reptiles, amphibians, and fishes), hematopoiesis takes place in the liver, the kidneys, and partly the blood vessels, where erythroblasts may be found together with mature erythrocytes. During the embryonic development of the higher, or warm-blooded, vertebrates (birds and mammals) and man, erythroblasts are formed in the vessels of the yolk sac and transformed into primary erythrocytes, which soon die. In man, they concentrate after birth in the bone marrow, where they develop from hemocytoblasts; it is only in pathological cases that erythroblasts are found in peripheral blood. The number of erythroblasts and their rate of reproduction increase in anemia. After the erythrocyte level in the blood is restored to normal, erythroblast activity in the bone marrow declines.