Day 15: The Respiratory System

Development of the Lungs

While a human baby is in its mother's womb, it does not need its lungs, it gets oxygen from its mother's bloodstream. In return it dumps the carbon dioxide from its cells into its mother's bloodstream. But what does the baby chick do inside its egg?

The egg shell is porous, meaning that gasses like oxygen and carbon dioxide can pass in and out of it freely. While the chick embryo is still only a few layers of cells, oxygen and carbon dioxide can still pass through the layers. But as the embryo grows, the layers of cells become so thick that oxygen from the outside cannot penetrate through to the inner cells. Long before the embryo becomes too large for oxygen to pass freely into its cells, the components of the circulatory system have begun to develop. Blood cells can be seen forming in the yolk around 28 hours. Vessels to carry the blood can be seen just a few hours later. And the heart begins to beat at 48 hours.

The lungs, however, are not used for respiration until just before the chick hatches. In the meantime, the chorioallantoic membrane, lying next to the inner surface of the shell acts as a lung. The capillaries of the chorioallantois exchange carbon dioxide waste from the embryo for oxygen which passes through the porous shell . Blood travels to and from the embryo to the chorioallantois through the allantoic vessels. As the embryo grows, the chorioallantoic membrane grows to meet the embryo's increasing need for more oxygen. By day 14 the chorioallantoic membrane covers the entire inside surface of the eggshell.

The amount of oxygen supplied through the eggshell is adequate until the chick starts the exhausting process of hatching. During incubation, the water lost by the egg through evaporation is replaced by air, which is stored in the air cell. By the time the chick begins hatching movements, the air cell has grown to fill 15% of the egg. The chick's first hatching movements are to break the shell membranes covering the air cell and take its first breaths with its lungs. The extra oxygen stored in the air cell is enough to allow the chick to break through the eggshell.

Even with the air cell ruptured, there is still a high concentration of carbon dioxide in the egg compared to the outside environment. The high concentration of carbon dioxide actually causes the neck muscles of the chick to jerk and thus allows the chick to peck through the eggshell.

The lungs of the bird begin developing early in order to be fully functional when the chick hatches. Lung buds branch off the trachea and can be recognized by day 4. Like a growing tree, they continue to branch into smaller and smaller bronchioles. Air chambers bud from the bronchii. Air sacs begin to bud from the ends of the mesobronchii by day 5. The air sacs continue to develop long after the chick has hatched.

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