The placenta is an organ (organ (anatomy)) that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mothers blood supply. Placentas are a defining characteristic of eutherian or "placental" mammals (Eutheria), but are also found in some snakes and lizards (squamata) with varying levels of development up to mammalian levels. The word placenta comes from the Latin for cake, from Greek (Greek language) plak�enta/plako�nta, accusative of plak�eis/plako�s - ????????, ???????, "flat, slab-like", in reference to its round, flat appearance in humans. Protherial (egg-laying) and metatherial (marsupial) mammals produce a choriovitelline placenta that, while connected to the uterine wall, provides nutrients mainly derived from the egg sac. The placenta develops from the same sperm and egg cells that form the fetus, and functions as a fetomaternal organ with two components, the fetal part (Chorion frondosum), and the maternal part (Decidua basalis).