The major site of hematopoiesis in the embryo is in the outer layer of the yolk sac which is derived from the extraembryonic mesoderm.
Hematopoietic stem cells migrate into the yolk sac fro primitive ectoderm or epiblast.
Hematopoietic stem cells leave the yolk sac and between the 4th and 5th weeks and start to populate the fetal liver.
Fetal hematopoietic stem cells subsequently migrate to the spleen, lymph nodes, thymus and bone marrow.
The liver is the major site of hematopoiesis in early embryonic life and then the spleen becomes the exclusive site of hematopoietic production until week 14 of gestation.
At weeks 15-18 the spleen is populated with T cell precursors and by week 23 the spleen is populated with B-cell precursors and form B-cell regions.
Once it is formed the fetal thymus becomes populated with lymphocytes from the stem cells of the yolk sac, liver and omentum.