The beginning of life is called conception. It takes place when the egg of the female is penetrated by the sperm of the male. This union between the sperm and the egg is known as fertilization. The entire process of conception in the human, as well as in other animals, is one of Nature’s great wonders. Let’s start from the very beginning:
As mentioned elsewhere, the male sperm are deposited in the vagina near the entrance of the cervix of the uterus or womb. Nature seems to have sensed that it would be a difficult trip for the sperm to bring about conception. For this reason, 100 to 200 million sperm are provided just for the purpose of fertilizing one female egg! The millions of tiny sperm, which can be seen only under a microscope, are so delicate that they live only a few minutes unless they are successful in passing through the cervix into the uterus.
The sperm have tails, called flagella, that push them forward. Actually, sperm look very much like miniature tadpoles, and they move forward like tadpoles by wiggling their tails from side to side. When they reach the cervix, the sperm must swim through a mucous barrier that covers the entrance to the inside of the uterus. Tens of millions of sperm are unable to do this, and are lost. Those sperm that pierce the cervix then swim up the three to four inches of the inside of the uterus to find the two exits at the upper ends where the Fallopian tubes begin. Tens of millions more sperm are lost before they get to the Fallopian tubes. Those that do survive swim into the narrow passageway of the Fallopian tube where they may finally meet an egg. But this meeting can take place only during two to three days of each month.
The egg leaves the ovary and enters the fallopian tube. There. It is fertilized by a single sperm. The fertilized egg begins to divide and to make many cells. When it reaches the uterus. some 3 to 4 days later, the fertilized egg buries itself in the wall of the uterus.
Females normally have one egg, no larger than the point of a pin, that leaves an ovary each month. This is called ovulation. Ovulation usually occurs halfway between two menstrual periods. (The menstrual period is described in chapter 21.)
When an egg leaves an ovary it finds its way to the funnel-shaped opening of the Fallopian tube. Nobody knows how it manages to get from the ovary to the Fallopian tube because an egg has no ability to move by itself. However, it gets there somehow. Once inside the Fallopian tube, the egg is very slowly swept down toward the uterus by tiny hairlike structures that line the tubes. These hairlike structures, called cilia, are so small they can be seen only under a microscope. It takes anywhere from three to five days for the egg to travel the three inches of the Fallopian tube, and during this time it may meet the sperm.
If an egg meets the sperm in the Fallopian tube, there is a good chance that one of the sperm will enter the egg and unite with it. This is called fertilization.
So, even if there are 100 million sperm that meet an egg in the Fallopian tube, only one will usually be able to pierce the outer coating of the egg and cause fertilization. When this happens, all the other millions of sperm die. Sometimes, however, two or more eggs emerge from the ovary. They, too, may be fertilized, each one by a different sperm, and the result will be twins or triplets or even more. Also, twins can result by the splitting of a single fertilized egg into two.
The male’s sperm, which is responsible for determining the gender of the embryo. contains approximately equal numbers of chromosomes for each sex. There Is, therefore, an equal chance of a couple having a boy or a girl, and approximately equal numbers of children of each sex are born.
But even when the egg and the sperm are united, it is not yet certain that a new life will come about. The fertilized egg must continue its journey for another few (lays down the remainder of the tube until it reaches the cavity of the uterus.
In order for pregnancy to take place, the fertilized egg must bury itself in the lining of the uterus at a spot where it can take root and grow. This is very much like what happens to a seed when it is planted. If a seed is planted in healthy soil with plenty of moisture and minerals in the ground, if it is watered often enough and in the sunshine frequently, and if the climate is not too cold or too hot, the seed will sprout and grow into a plant. In the same way, the fertilized egg must plant itself into a healthy lining of the uterus. If the lining is infected, or if there isn’t sufficient blood coming to that part of the uterus, the egg may no take root properly and will not grow. Then, even though it has been fertilized by a sperm, it will be prise out of the uterus into the vagina and pregnancy will end before it really begins.
With all the things that have to happen just right, it’s remarkable, isn’t. it, that so many millions of children are born every year!
Although we know a great deal about how life begins, there are still many processes that we know very little about. For example, no one knows why only one sperm out of hundreds of millions can penetrate an egg. What keeps the other sperm out? Also, no one knows how the egg chooses the particular sperm that enters it. Did you ever stop to think that if a different sperm had penetrated your mother’s egg you wouldn’t be you! You might: be your brother or your sister. Now, wouldn’t that be funny?