Every living creature requires a constant supply of food and of oxygen. The embryo is a living creature, and is no exception to the rule. It needs an abundant supply of easily assimilated food and of oxygen. When the hen's egg is first laid the entire contents, with the exception of the little light-colored disk which floats on the top of the yolk, form the nourishment. The disk alone is the living organism. In the earliest stages the embryo receives its food by simple absorption from the yolk. As the chick increases in complexity the yolk at first grows swampy, with fluid trickling here and there through the more solid portions. Thin walls form about these little streams, thus producing blood vessels which cover the entire surface of the yolk. These absorb the nourishment and turn it over to the embryo. As the latter grows in size both the yolk and white diminish. The embryo soon becomes larger than the remaining yolk and is attached to it by a cord filled with blood vessels which enter the chick near the center of its body. The abdominal wall has an opening at this point. One of the later occurrences in the life of the chick, before it breaks through the egg, is to have the last remnant of the yolk and its sac slip to the inside of the abdomen, which then completely closes over it. As yet, we have seen no arrangement for furnishing air to the chick. At the same point at which the blood vessels from the yolk enter the chick, another set of vessels pass in and out. These are attached to a large flattened bag which floats above the embryo against the upper side of the shell. This bag is called the allantois, and serves as a sort of lung for the developing chick. The shell is porous enough to allow air to pass through it. The blood vessels of the allantois take in oxygen and give out carbon dioxide through the porous shell. The blood thus altered is returned to the chick and serves its life purposes. One of the reasons why the chicken must turn its eggs in the nest is that, if the allantois remain too long in contact with the upper shell of the egg, it will become attached to it and will not thereafter perform its functions.