Ans. B. LiverFetal Hematopoiesisi. Begins in the yolk sac (2 -8 weeks) - progenitor cells migrate from the yolk sac to the liver at 5-8 weeks' gestationii. Only terminal differentiation of red cells occurs in the yolk saciii. Liver active from 5-8 weeks' gestation - mainly red cells. Spleen also involved before 20 weeksiv. Begins in the medullary cavity of the clavicle at about 10-12 weeks and in the medullary cavity of long of bones at 20 weeksv. gestationvi. Some lymphocytes are produced in lymph nodesvii. At term, all red cell production is in bones unless there is a reason for increased hematopoiesis