Yolk sac blood cells at E8.25-E8.5. Hematopoietic cells were visualized by Runx1/LacZ staining. (a) “Blood island” - like cell aggregations appear in the proximal yolk sac at around the start of blood circulation. The red arrow points to blood cells on their way through vitelline vein into embryonic vasculature. (b) Dense cell clusters in the proximal region of the yolk sac resist the circulation drive for some period of time. (c) In pre-circulation conceptus blood cells start to move distally before the yolk sac vascular plexus fuses with the embryo vasculature.