Embryology is the science that deals with the growth and development of an embryo. All birds lay eggs, which provide protection and nutrition for the developing embryo. Most people are familiar with chicken eggs because they are commonly found in our kitchens. Watching a fertile chicken egg develop can be an effective way to study embryology, investigate how organisms change and develop over time, and increase excitement and teamwork in the classroom.
Chickens are important farm animals for the production of meat and eggs. The breeds of chickens used for egg production have been developed through careful selection and crossbreeding to increase the number of eggs a hen will lay in a year. Hens that have not mated with a rooster will still lay eggs. These eggs have not been fertilized. They are not fertile and will not become embryos. Eggs sold in the grocery store are infertile.
A rooster mates with and fertilizes a hen’s egg before the hen lays the familiar hard-shelled egg. Under the right conditions, each fertilized egg will grow to be a chick. The yolk and albumen (the white of the egg) provide food for the growing embryo. The albumen also provides the embryo with protection against shock, cushioning the developing embryo in a water sac. The blunt end of the egg contains the air cell. Just before the chick breaks out of its shell, it takes its first breath of air from this air cell.
The egg provides much of what the developing embryo needs—food to grow, cushion from shock, and air to breathe. In mammals, the embryo develops inside of the mother’s body, which provides nourishment and protection. If birds grew heavy during pregnancy like mammals, it could inhibit their ability to fly. The main disadvantage to laying eggs is that they’re good to eat and vulnerable to prey. Although the egg provides energy and protection, the parent (the hen in the case of chickens) is still needed to keep the eggs warm and protect them from predators.
Eggs can be incubated in two different ways—by a broody hen or by an incubator. In the classroom, the incubator is the preferred method. There are two main types of incubators, still-air and forced-draft. Both types can be used in a classroom setting, although the forced-draft type is most common. Successful egg hatching requires maintaining optimal temperature, humidity, and ventilation, as well as regular egg rotation. These conditions can easily be met with proper use of an incubator.
Candling eggs is a simple and effective way to show students that there is life inside the eggshell. In a dark room, shining a bright light through the shell of a fertile egg will silhouette the developing embryo. After four days a heartbeat can be observed, and just a few days later the wings, legs, and a beak can be distinguished.
