Placentals are mammals that bear live young, which are nourished in the womb by a placenta. According to Dr. David Menton (a cell biologist and emeritus professor of human anatomy at Washington University Medical School in St. Louis), this amazing organ functions as the developing baby’s kidneys, lungs, digestive system, liver, and immune system. The placenta also protects “the developing baby from an attack by the mother’s immune system, since the baby and the placenta are genetically unique and distinctly different from the mother.” (David Menton, “The Placenta: a selfless servant,” Answers Magazine 2:1, 2007, p. 72.)
Offspring of placentals are born more fully developed than those of the marsupial mammals.