Page 12, Later Development, Develop. Physiol., Dr. D. Penney

Blood Cell Formation:

Erythrocytes are first formed in the yolk sac, from the 4th through 8th weeks of development (Table 3.08). The body mesenchyme and blood vessels are also sources of red cells during this early period.

From the 6th week until birth erythrocytes develop in the liver. The spleen and thymus also serve this function during fetal development. Finally, by the 4th month the bone marrow becomes active in erythropoiesis; this tissue is dominant in this function by the 5th month. Bone marrow carries this function on in adulthood.

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