Background Agricultural Connections
Eggs are a bit mysterious. The same egg a chicken lays to create the next generation of chickens is what you find in the grocery store refrigerator and take home to scramble. Students may worry that they are eating baby chicks; put these fears to rest! Eggs in the store are not fertile. The simple explanation of “not fertile” is that there are no developing chicks inside the eggs. A more complex explanation is that a rooster (male) needs to be present to fertilize the hen (female) to produce a fertile egg that will result in a chick. If there isn’t a rooster around, as is the case in grocery store egg production, the egg that the hen lays will not have the complete genetic information that is required for the first cell in the egg to begin to grow and divide.
Fertile eggs contain a living organism—an embryo that will be called a chick when it is fully developed. Embryology is a branch of biology that studies embryos (fertilized eggs) of biological organisms and their development. In a fertile chicken egg, a heartbeat can be seen by candling approximately three days after setting (the point at which the egg begins to be kept warm by a hen or incubator). Candling involves shining a bright light through the egg, allowing one to see the silhouette of the embryo that will become the chick—complete with beak and toes. While candling is truly exciting for adults and children alike, the experience leaves behind almost as many questions about what is going on inside the egg as it provides answers.
The chick embryo develops for 21 days. In that amount of time, the new chicken inside the egg will go from being a single-celled zygote (the first cell formed after fertilization) to a fully developed and functional chick that can peck its way out of the egg shell all on its own and walk, eat, and drink within minutes of hatching.
The old riddle, “Which came first, the chicken or the egg?” is a good starting point for talking about cycles. The question tries to get a person to decide where the beginning point is in a life cycle, but eggs come from chickens and chickens come from eggs, so it’s really the wrong question altogether! Instead of looking at the situation as having a beginning and an end, it is important to recognize that cyclical patterns exist in nature. This concept can be difficult for students who are more accustomed to considering short-term cause and effect situations. Presenting different life cycle examples as well as other examples of natural cycles (the water cycle, the cycle of seasons, etc.) can be helpful in getting students to think cyclically (something that returns to the beginning point) instead of linearly (something that has a definitive beginning/ending point).
