At the very beginning of its development the embryo breathes and nourishes itself exclusively via tissue diffusion. However, with its rapid increase in size following the implantation (stage 5, ca. 7-12 days), the required O2, nourishment and removal of metabolic wastes can no longer be ensured alone through diffusion. A shift to hematotrophic nourishment takes place. Already in stage 5b (ca. 9 days) the maternal blood begins to circulate in the lacunas of the syncytiotrophoblasts (lacunar stage). Soon afterwards a transport system also forms on the embryonic side with vessels and embryonic blood circulating in them.
At the very beginning of this hematotrophic nourishment stands the development of the erythropoietic system (vehicle for O2 transport) in the foreground. Only much later does the lymphatic system with the myelo- and lymphopoietic organs develop. They are responsible for protecting the body against infections and achieve their functional maturity only in the fetal period (the encounter with the microbial environment occurs only after birth). In addition the blood coagulation and fibrinolysis must also be developed so that hemostasis and the intactness of the vessel walls can be assured.