Illustration of the foetal surface of a twin placenta with separate umbilical cords. No septum is present across the equator and thus each placental component has vascular and metabolic communication. When both foetuses share a placenta this is termed a monochorionic placenta. The paired umbilical arteries (red) pass from the foetus to the placenta and are depleted in oxygen and nutrients. The umbilical vein (blue) supplies oxygenated and nutrient-rich blood from the placenta to the foetus. From Kollmann, J. (1907) Handatlas der Entwicklungsgeschichte des Menschen, vol 1, Fig. 175. G. Fischer, Jena.