The Amniotic Egg

Although amphibians were the first vertebrates to successfully invade land, they did not make a full transition to terrestrial life. They still require water in their environment to reproduce. Reptiles are considered the first fully terrestrial vertebrates because they do not need to reproduce in water, as most amphibians do. Reptiles produce amniotic eggs, which encase the embryo in a secure, self-contained aquatic environment. Amniotic eggs provide more protection for the developing embryo than do the jellylike eggs of amphibians.

Figure 41-6 shows the internal structure of the amniotic egg, including its four specialized membranes: the amnion, yolk sac, allantois, and chorion. The egg is named for the amnion (AM-nee-uhn), the thin membrane enclosing the fluid in which the embryo floats. The yolk sac encloses the yolk, a fat-rich food supply for the developing embryo. The allantois (uh-LAN-toh-is) stores the nitrogenous wastes produced by the embryo. Its blood vessels, which lie near the porous shell, function in the exchange of oxygen and carbon dioxide gases. The chorion (KAWR-ee-AHN) surrounds all the other membranes and helps protect the developing embryo. Protein and water needed by the embryo are contained in the albumen (al-BYOO-muhn). You are familiar with albumen as the egg white in a chicken's egg. In most reptiles, the leathery outer shell provides protection from physical damage, limits the evaporation of water from the egg, and allows diffusion of oxygen and carbon dioxide.

The amniotic egg first evolved in reptiles, but it also occurs in mammals and birds. The presence of this feature is strong evidence that reptiles, birds, and mammals evolved from a common ancestor. The eggs of some reptiles and nearly all mammals lack shells, and the embryo develops within the mother's body.

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Chorion Yolk sac figure 41-6

Chorion Yolk sac

Amniotic eggs have four major membranes. The tough but porous shell provides protection while allowing the exchange of oxygen and carbon dioxide. The amniotic eggs of reptiles and birds (shown here) are very much alike internally.

The skin of this spiny lizard, Sceloporus poinsetti, protects it from the rugged terrestrial environment and from water loss.

Watertight Skin

Because amphibians exchange gases through their skin, the skin must be moist and thin enough to allow rapid diffusion. A drawback of this kind of skin is that amphibians face the loss of body water through evaporation. Reptiles, such as the lizard shown in Figure 41-7, are covered by a thick, dry, scaly skin that prevents water loss. This scaly covering develops as surface cells fill with keratin, the same protein that forms your fingernails and hair. Lipids and proteins in the skin help make the skin watertight. The tough skin of a reptile not only helps conserve body water but also protects the animal against infections and injuries.

Respiration and Excretion

Modern reptiles have developed efficient respiratory and excretory systems that help them conserve water. All reptiles have lungs for gas exchange. All of the tissues involved in gas exchange are located inside the body, where they can be kept moist in even the driest environments. The excretory system of reptiles also helps them conserve body water. Land-dwelling reptiles excrete nitrogenous wastes in the form of uric acid which requires little water for dilution. Reptiles lose small amounts of water in their urine.

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