Most eggs usually have an oval shape with one end being pointed while the other is more rounded. The reason for the unusual shape is due to the egg being forced through the oviduct^注1 with the rounded end exiting first. During this process, the muscles of the oviduct contract behind the egg, forcing it to move forward. Since the egg's wall is still very soft and is still able to be shaped easily, this causes the pointed end to develop at the back side of the egg. This trait is likely to have come about due to evolution though natural selection.

The oval shape is very significant. A pointed egg will tend to sit on its side, with the rounded end tipping upward. The rounded end contains an air pocket, also known as an allantois^注2, and it has more tiny holes than the smaller end. Having the bigger end pointed upward improves the flow of oxygen to it and so this is where the head and brain will develop, while the tail will develop at the pointed end.

The structure and composition of the eggshell serves to protect the egg against damage and bacteria, and serves to (a)regulate gas and water exchange or the growing embryo^注3 within the egg shell.

When a hen lays an egg, allot the nutrients .for the embryo are already included within the shell. These all can be found in either the egg yolk or the white. The yellow part of the egg, also known as the yolk, is a food packet that feeds the developing embryo within the egg. Half of the yolk is water and the rest is a mixture of protein, vitamins and minerals that the embryo will need during development. The yolk also contains antibodies that were transferred from the mother hen to the egg yolk in order to protect the embryo from germs that may penetrate the egg. The embryo gets these nutrients through small blood vessels. An egg also contains a (b)transparent fluid, called the egg white, which consists of about 90 percent water, with the remainder mostly comprised of various proteins. The main purpose of the egg white is to protect the yolk against bacteria and provide additional nutrients for the embryo.

Apart from the yolk and white, only two things are originally missing in order for an embryo to develop: warmth and oxygen. Heat welcome from the hen sitting on the eggs, keeping them warm, in what is known as incubation. But, what about the oxygen and whet-e does it come from? Eggshells have thousands of tiny holes in the shell that allow the embryo to breathe. A hen's egg usually has around 7500 tiny holes. It is these small openings that enable oxygen to enter and carbon dioxide to leave. Oxygen, which can be found all around us in the air, enters the egg through the tiny holes in the cell. Once inside, the oxygen is taken in by a tissue called the allantois, which has plenty of small blood vessels that are connected to the embryo. With the help of the allantois, the embryo is able to exchange gases and liquid waste. Oxygen will be delivered through the blood to the embryo at the same time as carbon dioxide is removed from the embryo by the blood, and then released out of the shell. The waste products are stored in the hollow air pocket in the allantois. Only when the baby chick emerges from its shell will it be able to breathe on its own.

So the next time you have some eggs for breakfast, just remember how incredible eggs are.

• 注1:卵管
• 注2:尿嚢(鳥類では呼吸器官として機能する)
• 注3:胚

問 英文の内容から判断し、書き出しに続く ものとして最も適したものをそれぞれ選択肢1～4の中から選びなさい。ただし、(3)～(5)および(10)については質問に対する答えとして最も適したものをそれぞれ選択肢1～4の中から選びなさい。

• (1) It can be inferred from the reading that the rounded end
  • 1. has the most oxygen flowing to it.
  • 2. is more important than the pointed end.
  • 3. is just as important as the pointed end.
  • 4. is usually smaller than the pointed end.
• (2) One example of evolution through natural selection would be
  • 1. from an egg to a chicken.
  • 2. from a man to a chimpanzee.
  • 3. from an ape to a man.
  • 4. from a man to a woman.
• (3) Which of the following is the meaning of (a)“regulate” as used in the essay?
  • 1. change
  • 2. move
  • 3. arrange
  • 4. adjust
• (4) Which of the following is the meaning of (b)“transparent” as used in the essay?
  • 1. mirror
  • 2. flat screen TVs
  • 3. clear
  • 4. clouds
• (5) What is one of the purposes of the allantois within the eggshell?
  • 1. It stores food for the embryo.
  • 2. It helps the embryo breathe.
  • 3. It carries heat away from the embryo.
  • 4. It contains antibodies for the embryo.
• (6) It can be inferred from the reading that the egg yolk
  • 1. is just as important as the egg white.
  • 2. is more important than the egg white.
  • 3. is not as important as the egg white.
  • 4. is the same as the egg white.
• (7) From the reading, we can say the egg is pointed at one end because
  • 1. it is shaped by the hen's oviduct contracting.
  • 2. there is nothing inside the pointed end.
  • 3. it stops it from rolling away.
  • 4. it makes it easier for the hen to push it out.
• (8) Two important ingredients that are first missing from an egg are
  • 1. food and water.
  • 2. antibodies and food.
  • 3. proteins and oxygen.
  • 4. heat and air.
• (9) It can be inferred from the reading that, right after the baby chick emerges from its shell, it
  • 1. continues to make its own food.
  • 2. starts to depend on its shell for food.
  • 3. starts looking for its own food.
  • 4. removes the shell around it.
• (10) Which paragraph describes how the embryo breathes within the shell?
  • 1. the second paragraph starting with “The oval ・・・”
  • 2. the third paragraph starting with “The structure ・・・”
  • 3. the fourth paragraph starting with “When a ・・・”
  • 4. the fifth paragraph starting with “Apart from ・・・”