Mammals reproduce sexually, via the union of sperm and egg cells. Except for five species of egg-laying mammals, called monotremes, all mammals give birth to live young.
Mammals possess two main types of cells: somatic cells and sex cells. Somatic cells account for most of the body's cells and are responsible for growth, repair and healing. Sperm and egg sex cells contain one set of chromosomes each and fuse during sexual reproduction to form a zygote that later develops into an embryo and then a fetus. In most mammals, the fetus develops in the uterus until it is able to survive outside the mother's body. In the pouched mammals, called marsupials, offspring are born very early and finish development in the female's pouch. A placenta forms in some species of marsupials, while a simple yolk sac delivers vital nutrients in others.
Five species of mammals, called monotremes, actually lay eggs. Monotremes, including the duck-billed platypus and four species of echidna, are all native to Australia and New Guinea. Like birds, monotremes possess a cloaca, which is a single orifice that serves for both reproduction and excretion. The eggs receive nourishment from the female's body for a few weeks before they are laid. Like other mammals, monotremes have mammary glands and feed their young milk.