Question

What developmental adaptations in amniotes allowed for the colonization of land by vertebrates? Include the functions of the four extraembryonic membranes in your answer.

Answer 1

Amniotes are characterized by having an egg covered with an amnion, an adaptation to lay eggs on land or retain the fertilized egg within the mother. The embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians. The amniotes produce a terrestrially adapted egg, which is protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments.  Most mammals do not lay eggs (except for monotremes). Instead, the embryo grows within the mother’s body; however, even with this internal gestation, amniotic membranes are still present.

There are four layers or extraembryonic membranes-: the amnion, yolk sac, allantois, and chorion.

Amnion- The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus.

Yolk sac-  the yolk sac functions as a source of nourishment for the developing fetus.

Allantois- It helps the embryo exchange gases and handle liquid waste.

Chorion-  The function of the chorion is to contribute to the development of the placenta in placental mammals.