KEY CONCEPT Reptiles, birds, and mammals are amniotes.

Amniote embryos develop in a fluid-filled sac. The amniotic sac contains everything an embryo needs to grow. some develop inside mother’s body some develop inside a tough, semipermeable shell

The amniotic egg allowed vertebrates to reproduce on land. Allantois Holds waste materials as the embryo grows Embryo Amnion Protects and surrounds the embryo Chorion Allows gas exchange with outside environment Yolk sac Contains the nutrient supply for the growing embryo

Anatomy and circulation differ among amniotes. The first animals walked in a sprawl. Other amniotes, including dinosaurs, evolved a more upright stance.

All amniotes have two circuits of blood vessels. pulmonary circuit moves blood from the heart to the lungs systemic circuit moves blood from the heart to the rest of the body

Amniotes have a three- or four-chambered heart. reptiles hearts have three chambers birds and mammals hearts have four chambers THREE-CHAMBERED HEART FOUR-CHAMBERED HEART

Amniotes can be ectothermic or endothermic. Amniotes manage body heat in different ways. Ectotherms have body temperatures determined by the surrounding environment. Endotherms use metabolic heat to keep tissues warm. Endotherms can live in a wider range of climates than ectotherms.