The third membrane is a fluid filled sac called the amnion . It surrounds the embryo and the fluid it secretes cushions the embryo. The fourth and final membrane is the yolk sac . The sac contains the food for the embryo. A thick dry skin covered with scales made of a protein called Keratin .
Reptiles molt as they grow.
Reproduction is sexual with internal fertilization. Most are oviparous, while some snakes and lizards are either ovoviviparous or viviparous . Respiration occurs through well developed lungs, and in some turtles a moist cloacal surface. Reptiles are ectotherms. Being ectothermic enables an organism to survive on much less food than an endothermic organism.