This week I was offered and accepted a 1 pound freshly laid emu egg to scramble on the stove for breakfast. Immediately, I was stunned by the heavy weight and the beauty of the eggshell colored an intense blue-green color with a fine raised marbled texture.
For the first time in my life I wondered how a baby emu inside this firm, strong shell could breathe. I was told by Cheryl Burns, the emu owner, a simple but unbelievable answer; there are pores in the shell allowing oxygen to flow into the developing embryo and carbon dioxide to flow out, similar to lungs. Later, I discovered that the shell consists of calcium carbonate crystals (found in oyster shells and supplemental calcium pills) interspersed among protein fibers. Thousands of holes or pores existed in the shell. An experiment found in Scientific American for kids suggested submerging unfertilized hens eggs in a mixture of water, liquid detergent and food coloring for an hour which I did. As expected, upon cracking open three eggs, I found blue dye had stained the inside of the shell indicating that water had moved through hundreds of microscopic pores into the shell.
While store eggs are unfertilized, the development of a fertilized egg is a marvelous sequence of events. The male chicken, a rooster, mates by depositing sperm inside the hen under her tail feathers where they are stored for up to two weeks. A single one-celled egg is released from the hen's ovary to enter the lengthy, undulating oviduct. She adds a yolk, followed by albumen, the egg white. Fertilization occurs in the oviduct when the sperm and egg cell meet. The egg cell is encircled by a membrane after which the eggshell is added in the shell gland. Finally, 25-26 hours after the egg leaves the ovary, the egg is laid.
The emu egg in this image weighs 1¢ pounds, or 24 ounces, while the hen’s egg below weighs 2 ounces. Hens’ eggs provide sufficient nutrition to allow a chick embryo develop and also supply mankind with important nutrition in a tasty package.
Photo by Robert M. Ungerer
Development of a chicken embryo from a one-celled fertilized egg into a peeping walking chick is remarkable. The fertilized egg divides into two identical cells which each divide creating four cells followed by more divisions creating 8, 16, 32 and eventually millions of cells each with identical genes and chromosomes. A logical question arises, if all the cells have identical genes how do different organs and structures develop to create the chick? Biologists studying this enigma have discovered that chemicals from one part of the embryo stimulate nearby genes to activate development of limbs, a heart, the brain and all other structures from cells containing identical genetic material. Truly amazing in my opinion. Interestingly, during the first several days of embryo development in birds, fish and mammals, similar gill-like structures, limb buds and a tail appear.
Examination of a developing chick embryo reveals blood vessels growing over the yolk and albumen where nutrients are absorbed to promote growth. The heart pumps blood so oxygen is needed to supply heart muscle. Carbon dioxide a product of muscle action is released. Wastes are excreted into a sac. By 21 days the embryo has feathers, eyes, wings and is ready to hatch. Increasing carbon dioxide levels stimulate embryo neck muscles to contract making the head and beak move upward striking and cracking the shell allowing the chick to takes its first breath.
The unfertilized hen's egg, more than one trillion of which are laid per year worldwide, may be close to the perfect food. Just consider the fact that an egg provides all the nutrients required to create a mature chick from a single cell. Eggs contain vitamins, minerals, and all the amino acids to generate proteins. Unfortunately, eggs are notorious for their cholesterol content since one large egg contains about of the recommended daily human dietary consumption. On the plus side one egg provides of the recommended allowance of choline, an important nutrient needed by pregnant woman and nursing mothers for development of the fetal and newborn brain.
The common egg is marvelous. It provides nutrition for a baby chick to develop and is an important food source world wide. In moderation, enjoy eggs in your diet.