Amniotes
In amniotes - reptiles, birds, and mammals - the egg is enclosed with an outer layer, or amnion, which has led to further development of the oviduct. In reptiles, birds, and monotremes, the main part of the oviduct is a muscular tube, capable of considerable distension to transport the large eggs that are produced. This part of the oviduct is lined with glands that secrete the components of the egg white. The lower portion of the oviduct, or uterus, has a thicker layer of smooth muscle and contains the glands that secrete the egg shell.
In marsupials and placental mammals, the uterus becomes lined by an endometrium, and is more developed than in egg-laying amniotes. In many placental mammals, the uteri of each side become partially or wholly fused into a single organ, although in marsupials they remain completely separate. In mammals, the portion of the oviduct above the uterus is referred to as the fallopian tube.
Read more about this topic: Oviduct