Figure 38.19: The Mammalian Placenta In humans and other placental mammals, nutrients and wastes are exchanged between maternal and fetal blood in the placenta. This organ forms from both fetal tissue and tissues of the mother’s uterine wall. The embryo is attached to the placenta by the umbilical cord. Embryonic blood vessels invade the placental tissue to form fingerlike chorionic villi. Maternal blood flows into the spaces surrounding the villi, allowing nutrients and respiratory gases to be exchanged between the maternal and fetal blood.