Secrets Hidden in Eggs

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Eggs are common food to us. However, since eggs are living cells, a chick gets born out of a fertile egg like magic when the hen sits on it for just three weeks. Out of an egg that showed no sign of life, a living creature gets born. This mystery of life in eggs moves us.

An egg that has life also conceives a lot of secrets. First of all, let us take a look at the structure of the egg. At a glance, the inside of the egg looks very simple—yolk, egg white, and shell. In fact, however, the egg has quite a complicated structure embraced by several layers. The hard shell is mainly composed of calcium carbonate which is a common substance for limestone. When you peel an eggshell, you will see a thin, translucent membrane that might be a little annoying while peeling. This is a shell membrane which has two layers inside of the outer shell. The shell membranes look thin and weak, but they play an important role that adds durability to the outer shell. Inside the shell membranes, there are thin egg white and thick egg white that embraces the yolk which is surrounded by the vitelline membrane, and the yolk is fixed by the chalaza so that it can be located in the center. On one end of the shell membrane there is the air cell.

It looks like the eggshell has no hole at all, but it has about 7,000 pores through which a chick breathes. The eggshell is covered with a thin membrane called cuticle which controls breathing and protects the egg from germs from the outside. It is concentrated on the blunt part of the egg, which allows the air cell to be formed there. The air cell of a freshly laid egg is small, but it grows bigger as the gas that is formed inside the egg is excreted as time goes by.

Then which part of the egg, which is a cell, becomes a chick? Is it the yolk that is yellow just like the chick? The answer is neither the yolk nor the egg white, but the germinal disc. Inside the yolk surrounded by a membrane, there is a very tiny germinal disc that repeats cell division with nutrition it gets from the egg white and yolk and becomes a chick. Most eggs that we eat are infertile eggs that the hens lay by themselves, which will never produce chicks no matter how hard the hens sit on them because they don’t have germinal discs. Only the fertile eggs born through a rooster and a hen can produce chicks. Mammals can receive nutrition through the umbilical cord inside the mom’s womb, but eggs cannot do that. That is why the eggs contain all the nutrition that they need until chicks grow in them when the eggs come out of the hens.

When you take a close look at an egg, it’s not perfectly round or oval. Most eggs have a blunt end and a pointy end, which create a crooked oval shape. It is because the egg shape changes as it passes the oviduct when the hen lays the egg.

Eggs don’t keep rolling the same direction due to their unique shape. Instead, they bend toward the pointy side. On a flat surface, they also make one rotation and come back to the original position. It is because the center of gravity and the center of movement are different in the eggs due to their crooked oval shape. If an egg was completely spherical or balanced oval, it would roll away from the nest. However, because eggs are crooked oval, they can’t go far but stop or come back to their original positions. The shape of an egg is the best shape for the safety of the baby birds that are to hatch.

Even a little chick easily breaks and comes out of an egg, but if you hold the top and the bottom of an egg and squeeze it, it’s not easy to crush it even for a grown-up man. Some people perform an acrobatic movement of walking on eggs. You might think that those eggs will all break, but they easily bear one person’s weight. What is the secret to this great power hidden in these fragile eggs?

The outer shell is 0.3 ㎜ [0.01 in] thick and light, but it is strong enough to protect the chick from the outside pressure while the hen sits on it. Then how much weight can eggs bear? If you put four half-cut dome-like eggs on four sides and put books on top of them one by one, you will think that the weak egg shells will break easily, but the four egg shells can support over 12 ㎏ [26.5 lb], and four quail eggs can support over 4 ㎏ [8.8 lb], and ostrich eggs which are bigger and firmer can support even up to 200 ㎏ [441 lb].

There are many interesting points about the structural engineering of the eggs. It is because the cross section of an egg is an arch shape and it is a dome shape in three-D. The principle of the egg’s strength to endure heavy weight can be easily understood when we think of an arch-shaped stone bridge. The structure of the egg shell distributes the strength from the outside pressure instead of collecting it inside, just like the arch-shaped stone bridge. Thanks to this structural engineering of the egg, you need more strength than you think to crush it with one hand. But you don’t need to worry about a chick coming out of the egg. Eggs are weak in enduring power concentrated on one spot. That is how a chick can break the eggshell with its small beak and come out of it.

Since an arch and a dome, which are the shape of the egg, are stable to endure heavy weight, they have been often used in construction for a long time. The arch shape is easily seen in traditional Korean stone gates such as Namdaemun Gate and Heunginjimun Gate, in stone bridges in noble palaces, and in the tunnels or underground structures that need to endure the weight of the dirt. The dome structure which is strong against pressure is used in gyms or exhibition halls which require a big space without pillars.

The egg is perfectly fit for a chick to stay in peace and safety until it comes out to the world. As if someone had planned it and prepared it, the structure and shape of the egg is very scientific. It is small enough for us to hold in one hand, but mysterious secrets that we didn’t realize before are hidden in it.

Reference
Choi Jin, Fun Cooking & Delicious Science (in Korean, 신나는 요리 맛있는 과학), Sanchaek Junior, 2010
Gisela Lück, Egg Wisdom: Experiments around the Egg (in German, Eiweisheiten: Experimente rund ums Ei), Verlag Herder, 2005
Gwon Oh-gil, Life Symphony (in Korean, 생명 교향곡), Science Books, 2013
EBS Sci-teen, Egg Power: Dispersion of Force (in Korean, 달걀의 힘: 힘의 분산), EBS TV, Nov. 14, 2013