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Avian Extra-embryonic membranes Because the avian embryo has no anatomical connection to the hen, all of its nutritive requirements except oxygen must be contained in the egg. The embryo very early in its development develops special membranes external to its body to access the nutrients in the egg and to carry out essential body functions. There are four of these special membranes and their names and functions are as follows: Yolk sac: This sac envelops the yolk and produces an enzyme that changes the yolk material to a form that can be used as a food source by the developing embryo. Any remaining, unused yolk material in the yolk sac when the chicken hatches from the egg is drawn into the abdomen for use by the chicken for the first two to three days after hatching while the chicken learns what to eat/drink and where to find it. Amnion: The amnion forms a sac that is filled with fluid in which the embryo floats. In this way it provides a shock-absorbing environment in which the fragile embryo can develop without harm from normal day to day knocks. Allantois: The allantois develops an extensive circulatory system connected to that of the embryo and driven by the new embryonic heart. When the allantois is fully developed it completely surrounds the embryo. This membrane has a number of functions: Respiratory - the developing embryo uses oxygen and produces carbon dioxide i.e. it has respiration. It is unable to carry out this function itself and hence the allantois oxygenates the blood and eliminates the carbon dioxide. Excretory - it removes the wastes that result from the embryo’s metabolism and deposits it in the allantoic cavity. Digestive - it provides the means for the embryo to access the albumen and the calcium of the shell. Chorion: The Chorion fuses the inner shell membrane to the allantois and helps that membrane to carry out its functions.