Egg-laying mammals are rare, and are called monotremes (order monotremata). Marsupials are mammals that bear live young who continue to develop in a pouch. Most mammals are placental (such as humans), with live births and a placenta that facilitates nutrient and waste exchange between the mother and the fetus. Prokaryotes are microscopic single celled organisms lacking distinct nuclei and organelles. Autotrophs are organisms who make their own food (organic raw material) from inorganic energy sources.