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TLDR: The allantois is a structure that helps developing embryos exchange gases and handle waste. It is part of the extraembryonic membranes that identify amniotes (mammals, birds, and reptiles) and has played a role in their transition from water to land.

The allantois is a sac-like structure filled with clear fluid that forms part of the developing embryo in amniotes. It is one of the extraembryonic membranes, along with the amnion, chorion, and yolk sac. These membranes help amniotes survive and develop in their environment. The allantois specifically helps with gas exchange and waste management.

In mammals, the allantois is one of the fetal membranes and is involved in the development of the umbilical cord. It collects liquid waste from the embryo and exchanges gases used by the embryo. The characteristics of the allantois can vary among mammalian species, including differences in size and shape.

In marsupials, the allantois is avascular, meaning it has no blood vessels. However, it still serves the purpose of storing waste. Some marsupials, like koalas and wombats, demonstrate fusion of the allantois with the chorion.

In reptiles and birds, the allantois evolved as a reservoir for waste and a means for oxygenation of the embryo. Oxygen is absorbed by the allantois through the eggshell. In most reptiles and birds, the allantois consists of extraembryonic endoderm enclosed in mesodermal tissue.

The allantois has clinical significance as well. During embryonic development, it protrudes into the urogenital sinus and later becomes the urachus, a duct between the bladder and the yolk sac. A patent allantois can result in a urachal cyst. Additionally, the allantois can be cultured ex vivo and is used as a model system for studying blood vessel formation and drug screening.

So, the allantois is an important structure in the development of amniotes, helping with gas exchange and waste management. It has different characteristics in different species and plays a role in the formation of the umbilical cord in mammals. It has clinical significance and can be used for research purposes.

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