In the northern continents the placental method of mammalian reproduction evolved with many ensuing benefits. The placenta allows the young to remain within the uterus for a very long time. It is a flat disc that becomes attached to the wall of the uterus and is connected by the umbilical cord to the foetus. The junction with the uterine wall is highly convoluted so that the surface area between the placenta and the maternal tissues is very great. It is here than that the interchange between the mother and foetus takes place. Blood itself does not pass from mother to young, but oxygen from her lungs and nutrients derived from her food both dissolved in her blood, diffuse across the junction and so enters the blood of the foetus. There is also traffic in the other direction. The waste products produced by the foetus are absorbed by the mother's blood and then excreted through her kidneys.