The most plentiful of most mammals are the placentals. Unlike the embryos of marsupials, the embryos of placental mammals are nourished within the mother's body by a special organ, the placenta, and are not born until development is well advanced. Placenta means "flat cake", which is the shape of this organ in humans; it is an evolutionary development of the membrane which in the reptilian ancestors of mammals enveloped the embryo within the eggs. In placental mammals these membranes have become connected to the wall of the uterus, so that the embryo can obtain nourishment directly from the body of the mother. This adaptation freed the embryo from dependence on the limited food supply of the egg. It enabled the young animal to remain within the protective environment provided by the mother's body for a much longer period until its complex brain and other systems have had a chance to mature.