Describe the structure and function of the four extraembryonic membranes found in reptiles, including birds. Are these four present in placental mammals? In what ways are their roles similar in reptiles and mammals? How do they differ?

The chorion is a vital extraembryonic membrane that envelops the entirety of the embryo and its associated membranes. Its primary role is to facilitate gas exchange, serving as a respiratory organ for the developing embryo. In reptiles and birds, the membrane is located just beneath the hard or leathery shell and allows oxygen to reach the embryo while permitting the release of carbon dioxide and other waste gases. An interesting aspect of the chorion is its structural adaptation. In reptiles and birds, it fuses with the allantois to form the chorioallantoic membrane, which further participates in respiration and excretion.

In contrast, in placental mammals, the chorion develops finger-like projections called chorionic villi that invade the lining of the uterus. This interdigitation leads to the formation of the placenta, a complex organ that facilitates nutrient uptake, waste elimination, and gas exchange through the mother's blood supply. This structural adaptation underscores a significant functional evolution from a simple respiratory membrane to a key component of the placental system.

Positioned closer to the embryo than the chorion, the amnion is a clear, thin membrane that forms an enclosed sac filled with amniotic fluid. The amnion plays a critical protective role in embryonic development. The cushion of amniotic fluid guards the embryo against physical shocks and injuries while maintaining a consistent and congenial environment for the embryo's growth.

Additionally, the amnion keeps the embryo from dehydrating while preventing adhesion to surrounding tissues. This allows for free movement, which is important for muscle development and bone growth. The presence of the amnion is a notable evolutionary advancement as it enables the embryo to develop in a watery environment, similar to that which life originally evolved from, thereby providing an aquatic environment on land.

The yolk sac, initially the primary source of nutrition for the embryo, contains yolk, which is rich in proteins and fats necessary for development. In reptiles and birds, the yolk sac is a dominant feature due to its size and function. However, in mammals, the sac is substantially reduced as the embryo derives nourishment directly from the mother.

The allantois emerges as a pouch from the hindgut of the embryo and serves dual functions. It stores the metabolic waste products from the developing embryo, and in birds and reptiles, it also aids in gas exchange. The allantois grows to eventually fuse with the chorion, increasing the surface area for nutrient absorption and gas exchange, thereby forming the chorioallantoic membrane. This membrane acts as a rudimentary lung, sustaining the embryo with oxygen.

Reptiles, birds, and mammals share the same set of extraembryonic membranes, but the form and function of these membranes can be distinct. In reptiles and birds, the chorion and allantois combine to form the chorioallantoic membrane, which lies next to the eggshell and is principally responsible for gas exchange and waste collection. The yolk sac is large and serves as the primary source of nutrients. In contrast, in placental mammals, the chorion substantially contributes to placenta formation, facilitating direct nutrient and waste exchange between the mother and embryo. The yolk sac is present but greatly reduced and mostly vestigial, having lost its role as a primary nutrient source.

These differences highlight a remarkable aspect of evolutionary adaptation. Reptiles, laying eggs in variable external environments, have a greater reliance on self-contained nutrient supply and waste management systems. Placental mammals, however, have evolved intricate connections with the mother, providing a stable uterine environment for development and more efficient nutrient and gas exchange mechanisms. This evolutionary transition underscores the adaptive strategies that different classes of vertebrates employ to ensure the successful development and survival of their offspring.