The sites of hematopoiesis in the developing embryo in the correct temporal order is which of the following?

a. yolk sac, fetal liver, bone marrow

b. yolk sac, aorta, fetal liver

c. fetal liver, bone marrow

d. yolk sac, placenta, bone marrow

Answer: A.

Explanation: During embryogenesis, there are several sites for hematopoiesis. In week 3 it begins in the Yolk Sac, in Pander's Island or Wolff's Island. In week 5 hematopoiesis starts on the fetal liver and until week 12 hematopoiesis begins in the bone marrow, spleen, and thymus.