Amniotic Egg- Novel Adaptation

Evolved for terrestrial adaptation:

-Enclosed in protective shell

-Albumen fluid for protection and protein

-Four extraembryonic membranes

-Provides embryo with an internal aquatic environment

Four membranes:

~Yolk sac

Surrounds yolk

~Amnion

Fluid-filled sac surrounding just the embryo

~Chorion

Flexible membrane surrounding embryo and all other membranes

Facilitates gas exchange

~Allantois

Stores nitrogenous waste

Facilitates gas exchange

Outside to inside layers:

-Outermost layer is water-tight, protective shell:

  • Reptiles: leathery

  • Birds: hardened made of calcium carbonate

-Albumen between the shell and chorion

  • Provides embryo with water, protein, and cushioning

-Chorion

  • Extraembryonic membrane surrounding the embryo, yolk sac, and allantois

  • Develops from folds in the embryo’s body wall

  • Functions in gas exchange between the embryo and external environment

-Amnion

  • Fluid-filled cavity providing embryo with internal aquatic environment (cushions embryo from shock, hydrates)

  • Develops from folds in embryo’s body wall

-Yolk sac surrounding yolk

  • Extension of midgut

  • Blood vessels in yolk sac transports yolk nutrients to embryo’s circulatory system

-Allantois (gets smaller as embryo grows)

  • Stores nitrogenous waste

  • Extension of hindgut

  • As embryo grows, it fuses with chorion to facilitate increased need for gas exchange

Mammals and derived traits:

Egg is replaced by a gestational pregnancy:

  • Amnions are still present as the amniotic sac and fluid

  • Monotremes retain egg-laying trait

Placenta and umbilical cord are derived from yolk sac and allantois

  • Functions in nutrient transport and gas exchange (waste and feeding)

Chorion forms most of the placenta, mother’s uterine lining forms rest of placenta

All amniotes have novel derived traits:

-Waterproof skin

-Keratinized structures (nails, claws, horns, etc,)

- Costal respiration

  • Inhalation/exhalation produced by movement of ribs and rib musculature

Evolution of amniotes:

-Anapsid: no temporal hole, present in early amniote groups that are now extinct, present in turtles, but due to secondary loss of temporal holes “functionally anapsid”

-Diapsid: two temporal holes, present in birds and reptiles

-Synapsid one temporal hole, present in mammals and extinct therapsids