26.1 Amniotes
KEY CONCEPT
Reptiles, birds, and mammals are amniotes.
26.1 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
26.1 Amniotes
• The amniotic egg allowed vertebrates to reproduce on
land.
Embryo
Allantois
Holds waste materials
as the embryo grows
Amnion
Protects and
surrounds the
embryo
Chorion
Allows gas exchange
with outside environment
Yolk sac
Contains the nutrient
supply for the growing
embryo
26.1 Amniotes
Anatomy and circulation differ among amniotes.
• The first animals walked in a sprawl.
• Other amniotes, including dinosaurs, evolved a more
upright stance.
26.1 Amniotes
• 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
26.1 Amniotes
• 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
26.1 Amniotes
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.