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Alive & KickingThe Chicken and the Egg |
There is nothing common about the common egg-be it frog, chicken or mammal. An egg is carefully loaded for success. And the hatching of a chicken egg, considering the physical and behavioral habits of the hen, makes the hatching one step short of a miracle.
Being a mother hen is not a simple task! Try incubating an egg for 21 days! (An armpit might work well.)
You can imagine the joy I felt seeing my bantam hen emerge from her well-hidden nest with her first brood of twelve two-legged, fluffy, down-covered chicks. Each was the size of a golf ball, but with legs.
Mother hen was clucking and patiently instructing them on things to peck. As I came close to her, she sounded the alarm and they scampered under her wings, protected and warm.
Which did come first, the chicken, or the egg?
My high school students and I worked experimentally with both chicken and frog eggs. Most of our efforts were at the observation level.
Frog eggs will continue to develop after they are removed from a pond if they are placed in thin plastic sacks, which are easily suspended in classrooms or refrigerators. However, development usually stops before hatching; we suspect wastes accumulate and bacteria or mold growth usually take over.
A careful student can make a small dime-sized window in a developing, fertile hen's egg shell and shell membrane, which will expose the developing embryo. For best results, the window is covered at once by a thin piece of glass glued to the shell with melted paraffin. The embryonic tissue early on will appear like a light spot floating on the yolk and by 48 hours the heart will begin to beat. In the classroom, we have had embryos develop up to a week with this technique. Again, problems with maintenance of a sterile environment will result in cessation of development.
As I think about the problems of a real live egg (not an Easter egg-rabbits lay those), I hold profound respect for the process that must take place.
. Each chick has two parents.
. The sterile, fertilized egg is encased by a shell to resist invasion by bacteria and mold.
. The shell allows O2 and CO2 to exchange through its pores.
. Moisture is stored mostly in the egg white and the setting hen's moist skin slows water loss from the egg. If too much water is lost, the chick will die.
. The successful egg must contain a 21-day supply of nutrients, which are mostly stored in the yolk.
. The shell provides both protections from some predators and the motion of the hen's body as she broods the egg.
. The shell must not be too strong for the chick to escape during hatching.
. A mature hen, influenced by spring's longer daylight, will average laying one egg a day. She forms her clutch of eggs over many days (12 to 20 eggs, usually). If you take her eggs as she lays them, she will not stop laying one a day for many days.
. During the 21 days of incubation the hen will eat little, and leaves the nest for short periods only. Egg temperature must stay close to 100 degrees F or the chicks will die.
. Within 48 hours of incubation, the beating heart can be seen and the transport of vital materials continue as cells grow and divide.
. Soon after, the head, eye and spinal structures become apparent.
. Within the developing chick, not only its own body structures are being made, but its reproductive systems are forming to provide for the next generation. All living things have a life span, they die: no next generation, no more chickens.
. During the last period of incubation, the hen must turn the eggs. She rolls them with her beak. If not turned, the chick will not develop normally.
. The eggs must all hatch within a day or so. Although the clutch is laid at a rate of one egg a day, egg incubation does not start until the clutch is complete. All eggs then begin development at the same time so that they all hatch about the same time.
. Thoughtfully, the egg contains about 2 days of extra food while the chicks learn to eat - this is when they're shipped to feed mills all over the country. In the last days of incubation, a sack within the egg called the yolk sac is pulled through the body wall containing this post-hatching food (the remaining yolk).
. A temporary structure called the yolk tooth forms on top of the chick's upper beak, which wears a hole through the shell. The chick must work its way out of the shell without assistance from the hen. Students, if not warned, will always try to help the struggling chick hatch, usually with disastrous results.
. Once the egg is holed, the chick's lungs inflate, breathing starts and peeping can be heard.
. When a hen lays an egg, there are great exclamations of joy by all the hens in the hen house and by the hen herself-"Look what I did!"
. The new chicks need the mother hen to educate them and protect them. Every child should have the experience of watching a mother hen and her chicks!
Now, maybe it's best if we let the chicken make the egg. Now, who made the chicken?
Have a Happy Easter!
Guest columnist John Cooke taught high school biology for 30 years and is pleased to share his insights with our readers.