Fertilization occurs about 24 hours before the egg is laid. A sperm cell, actually a number of sperm cells, penetrates the blastodisc on the yolk, and a new life begins. When the egg is laid, the blastoderm contains several hundred cells. When the egg cools, embryonic development stops. Embryonic development starts again when the temperature is increased. If the temperature is then decreased to room temperature a second time, the embryo will die.
Prior to incubation, all cells in the embryo look the same and have the same function. That changes, however, during the first 4 days of incubation. As cell division continues, the cells receive messages that cause them to become different types of tissues and organs. Some cells will develop into the circulatory system, others into the brain, and still others into muscles. The process in which similar cells are instructed to take on different functions is called differentiation. The first 4 days are a time of dramatic change. Mistakes sometimes occur in this process. If it is a serious mistake, the defect is lethal and the embryo dies. In the incubation process, these mistakes cause "early deads." If the biological mistake is not as serious, the embryo may develop longer before dying, or it may survive with a congenital defect.
While the embryo is developing, temporary structures are formed to support life. They are called extra-embryonic, because they do not become a part of the embryo. The one that is visible at the earliest stage of development is the amnion. It looks like a small pool in which the embryo is floating. Its purpose is to protect the embryo. The allantois is a structure that collects the waste that the embryo produces before it hatches. Many of the nutrients that the embryo needs are removed from the yolk by the yolk sac. Its blood vessels can carry nutrients from the yolk to the embryo. The last structure is the chorion, which lies between the embryo and the shell. An important function is for it to exchange the carbon dioxide produced by the embryo and bring oxygen to the embryo. The chorion also is important for dissolving mineral from the shell and making it available to the chick for forming bones.
Each day of incubation, a developing embryo looks a little different (Figure 4). Some of the visible changes are listed in Table 5. Beginning at day 18, several major changes occur. One is that the embryo pokes its head through the inner membrane into the air cell, then begins to breathe. It also takes any remaining yolk into its abdomen through the navel. The chick must then break out of its shell. These are major hurdles for the chick, and a number of chicks die before completing the tasks.
If eggs are incubated late Wednesday afternoon, most of the visible hatching will be on day 20 and 21, Tuesday and Wednesday. When a chick breaks out, it punches a small hole in its shell (pipping). Then it continues the process all the way around the shell. When the circle is almost complete, the large end of the egg can break away easily. Pushing off with its legs, the chick comes out if its shell. Several more hours are needed before the chick dries and can get up and walk. By Thursday morning chicks should be dried and fluffy.
Table 5. Time that prominent features appear in a chick embryo.
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