The human spleen arises in week 5 within the dorsal mesogastrium as proliferating mesenchyme overlying the dorsal pancreatic endoderm.
Cells required for its hemopoietic function arise from the yolk sac wall and near dorsal aorta.
The spleen generates both red and white cells in the 2nd trimester. Many embryonic RBCs remain nucleated.
D4 Dorsal Mesogastrium (Carnegie stage 13)]]
Reference: https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Spleen_Development