Reptiles

Characteristics of Amniotes

The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, outside of the chorion provides the embryo with water and protein, whereas the fattier egg yolk contained in the yolk sac provides nutrients for the embryo, as is the case with the eggs of many other animals, such as amphibians. Here are the functions of the extraembryonic membranes:

  1. Blood vessels in the yolk sac transport yolk nutrients to the circulatory system of the embryo.
  2. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment.
  3. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration.
  4. The amnion protects the embryo from mechanical shock and supports hydration.

In mammals, the yolk sac is very reduced, but the embryo is still cushioned and enclosed within the amnion. The placenta, which transports nutrients and functions in gas exchange and waste management, is derived from the chorion and allantois.

Art Connection

The illustration shows an egg with the shell, embryo, yolk, yolk sac, and the extra-embryonic membranes
An amniotic egg. The key features of an amniotic egg are shown.

Which of the following statements about the parts of an egg are false?

  1. The allantois stores nitrogenous waste and facilitates respiration.
  2. The chorion facilitates gas exchange.
  3. The yolk provides food for the growing embryo.
  4. The amniotic cavity is filled with albumen.

Additional derived characteristics of amniotes include a waterproof skin, accessory keratinized structures, and costal (rib) ventilation of the lungs.