During which of the following prenatal stages, are leukocytes not present in the developing embryo?

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.