Quiz 26: Amniote Origins and Nonavian Reptiles
The four membranes associated with the amniotic egg are discussed below- • Amnion- It encloses the embryo in fluid, providing aqueous medium for the growth of the embryo. The aqueous medium serves as a cushion around the embryo. • Allantois- Metabolic wastes of embryo are stored in allantois. It is highly vascular and acts as a respiratory organ. • Chorion- It is vascularized and surrounds the entire content of the egg. It also acts as a respiratory organ like allantois. • Yolk Sac- It stores nutrients for the nourishment of embryo. In placental mammals and marsupials, yolk sac do not contain yolk but forms a placenta between embryo and mother for the exchange of gases and nutrients and removal of wastes generated by the growing embryo.
Amniotes have skin that is more keratinized than early tetrapods. Their skin is also less permeable to water as compared to their ancestors. Amniotes draw air into the lung by expanding the thoracic cavity with the help of coastal muscles while early tetrapods used to fill their lungs by pushing air from oral and pharyngeal cavities in to the lungs.
Amniotes are divided in to three groups namely- • Anapsids • Diapsids • Synapsids The living amniotes which have originated from these 3 groups are: • Turtles originated from Anapsids • Birds and reptiles except turtle originated from diapsids. • Mammals have originated from synapsids.