In mammals, a tissue formed within the uterus through which nutrients are passed from the mother to the embryo (and later the fetus) and its wastes are removed. It is analogous to the protective membranes in the egg of other amniotes. placental n. A mammal that gives live birth to well-developed young that have prolonged embryonic development within the mother’s uterus. Marsupial mammals also have a placenta, but the embryo spends less time developing in the uterus before birth. Placentals include animals as diverse as humans, elephants, dogs, and mice.