- Most reptiles and all birds enclose their eggs in a shell. Within the shell, extraembryonic membranes take form. Two saclike membranes—the inner amnion and the outer chorion—are produced by the mesoderm of the embryo. From the end of the embryo’s digestive tract the allantois, an endodermal sac, grows between the amnion and the chorion.
- The amnion encloses a fluid-filled chamber within which the embryo grows. The allantois serves as a receptacle for embryonic wastes. Its outer wall fuses with the chorion. The chorion of birds and reptiles, linked with the embryo by blood vessels, lies adjacent to the inner surface of the porous egg shell. Oxygen passing through the shell is picked up by the chorionic blood vessels and carried to the embryo. Carbon dioxide received from the embryo is dissipated through the shell. Vessels in the yolk sac, formed from endoderm and mesoderm, carry digested food to the bird or reptile embryo.
- Except for such primitive egg-laying mammals as the platypus and the echidna, mammalian embryos develop within the mother’s uterus. The fertilized mammalian egg, like the eggs of reptiles and birds, develops an amnion, a chorion, an allantois, and a yolk sac. The embryo floats in the protective amnionic fluid.
- The chorion, with its many blood vessels, lies close to or fuses with the lining of the mother’s uterus to form the placenta. The allantois is incorporated into the umbilical cord connecting the embryo with the placenta. Only a little food is supplied to the mammalian embryo by the yolk sac. The embryo’s principal supply of food and oxygen passes from the uterus through the placenta’s thin walls to the blood vessels of the chorion.
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