A chicken embryo needs nutrients, water, oxygen, and the proper temperature and humidity to develop into a healthy chick. An egg contains most of what a chick needs for survival while developing within the shell. Identifying the parts of a chicken egg and their functions can help students understand how a chicken embryo survives and develops.
The egg’s shell has more than 7,000 tiny pores that allow oxygen to pass into the egg and carbon dioxide to pass out. The inner and outer membranes, found between the eggshell and the egg white, keep bacteria from entering the egg and help to slow evaporation of moisture from the egg. The air cell, located between the inner and outer membranes at the large end of the egg, holds oxygen for the chick to breathe. The pores in the eggshell are larger and more numerous at the large end of the egg. This allows oxygen to enter the air cell easily. Just before hatching, the chick uses its egg tooth to puncture the air cell, which will provide about six hours of oxygen for the chick while it pecks its way out of the shell. The albumen (al byoo min), or the egg white, cushions the egg yolk floating within it and is the main source of protein and water for the embryo. The yolk provides food for the embryo. It is made up of fats, carbohydrates, proteins, vitamins, and minerals. The chalazae (kuh ley zee) are cords on two sides of the yolk that keep the yolk floating in the center of the albumen. Chalaza (kuh ley zuh) is the singular form. The germinal disc is the white spot on the yolk. This is where the female’s genetic material is found. When the egg is fertilized, the germinal disc becomes known as the blastoderm. This is what develops into a chick during incubation. As the embryo develops, blood vessels attach to the yolk to access nutrition and to the inner membrane to access oxygen and release carbon dioxide.
Typically, the mother hen lays one egg per day until she has completed a clutch of about 8–13 eggs. If the eggs have been fertilized by a rooster, chicks may grow and develop inside given the right conditions. Once she has a full clutch, the mother hen begins brooding, only leaving the nest at dawn or dusk to feed. Because the hen’s body temperature is generally 105–106°F, she is able to heat the eggs to 100–101°F by sitting on them. Incubators can substitute for the warmth of a hen, but must maintain the proper temperature; the incubator should hold 99–102°F.
Maintaining the correct humidity ensures that the air cell does not become too big or too small. The drier the outside air, the faster the fluid inside the egg evaporates and the faster the air cell grows. This causes the inner membrane to dry out and stick to the chick, which prevents the chick from turning inside the shell. If the humidity is too high, the chick may drown in excess fluid after it breaks into the air cell. Humidity can be measured using a wet-bulb thermometer. Relative humidity should be 50–55% for the first 18 days and then 65% for the last 3 days. Humidity can be maintained by adding the correct amount of water to the water tray that is included in most incubators; if an incubator does not have a built-in tray, simply place a shallow pan of water in the bottom.
It is important for the eggs to be turned so that the embryo does not adhere to the wall of the egg. The hen turns the eggs by using her beak to scoop under the egg and roll it. If an incubator is not equipped with an automated egg turner, the eggs must be turned manually. Eggs should be turned at least three times a day for the first 18 days.
Use the