What is the blood-rich tissue that lines the walls of the uterus during pregnancy and nourishes the embryo?

The blood-rich tissue that lines the walls of the uterus during pregnancy is called the placenta. It forms from the outer layer of the embryo and the uterine tissue, facilitating the exchange of nutrients, gases, and waste between the mother and the developing fetus. The placenta also produces hormones that are crucial for maintaining pregnancy.