This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
During which of the following prenatal stages, are leukocytes not present in the developing embryo? 
Select and explain. 
A. Hepatic stage, B. Bone marrow stage, C. Mesoblastic stage, D. Splenic stage
C: Mesoblastic stage
Answer: C: Mesoblastic stage 
Explanation: Leukocytes are absent during the mesoblastic (yolk-cell) stage, but appear at approximately two months gestation in the liver and somewhat late in the spleen. The bone marrow first paicipates in hematopoiesis at approximately six months gestation and assumes an increasingly larger role thereafter. The liver and spleen cease to paicipate in hematopoiesis at about the time of bih, and this task is completely taken up by the red bone marrow. Ref: Kaushansky K. (2010). Chapter 16. Hematopoietic Stem Cells, Progenitors, and Cytokines. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e.