Developing bird foetus in egg, cutaway illustration. This egg (from a chicken) has a yolk (yellow) and egg white (albumen), enclosed in an egg shell (white). The egg shell and white are found in reptile eggs as well, but are absent or different in mammals. The developing foetus is surrounded by an amniotic sac and amniotic fluid, encased in a membrane called the amnion. The foetus is nourished by the yolk, contained in its yolk sac. The egg includes the allantois structure (lower right, translucent) to exchange gases and handle liquid waste. Surrounding the yolk is another membrane, the chorion. For a bird egg at an earlier stage, after fertilisation, see image C040/3984. |