4.1.1 THE MALE SEX CELL: SPERM
Human life is reproduced sexually, i.e., by the combined effort of two individuals of different sex: a male and a female. In order to reproduce, both the male and the female must be sexually mature, i.e., their bodies must have developed to the point where they produce sex cells (gametes).
The male sex cell is called sperm (spermatozoon, plural: spermatozoa). Sperm cells are produced in the male sex glands (testicles). They are the smallest cells in the human body. While they are invisible to the naked eye, they can be seen and studied under the microscope. An individual sperm cell has a certain vague resemblance to a very skinny tadpole, and it is composed of three main units: head, body, and tail. Its total length is about 1/600th of an inch. The head, which is only 1/10th of the length of the sperm, contains twenty-three special bodies (chromosomes) carrying the man's inheritable characteristics. One of these chromosomes determines the sex of the child that might be produced if this particular sperm should unite with a woman's egg. In other words, sperm cells come in two varieties: those (containing an X-chromosome) that will help produce girls, and those (containing a Y-chromo-some) that will help produce boys. Behind the head is the body, which plays an important part in obtaining the energy that enables the sperm to move. The movement itself is caused by the vigorous, whip-like lashings of the tail. The sperm can thus move ahead at the rate of 1/8th of an inch per minute under favorable conditions. This movement is essential since the sperm has to reach the egg deep inside the woman's body (i.e., in one of the Fallopian tubes).
Once the male sex glands have matured during puberty, they produce millions of sperm cells every day. Obviously, only the tiniest fraction of that number can ever contribute to actual conceptions. In the course of a man's life, most of his sperrn cells are ejaculated under conditions that offer no chance for a fertilization. Those that are not ejaculated are simply reabsorbed into the body.