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workings of his penis, outwardly, he is at least on somewhat good terms with that part of his anatomy. But when it comes to testicles, too many men know almost nothing about them.

      Be forewarned: By the time you’ve finished with this chapter, you will not only be seeing testicles differently, you’ll also be feeling them in a whole new way.

      Making the descent

      As a baby boy develops inside his mother’s womb, his testicles are still inside his body (in his abdomen). During the last few months before birth, the testicles poke their way outside, or descend, into the scrotum, a sac of skin located at the base of the penis. Occasionally, one or both of the testicles don’t make the descent.

      Some of these undescended testicles are of the hide-and-seek variety, meaning that, during the first year or so, they kind of come and go. As long as they make an occasional appearance, everything will be just fine, and eventually they’ll get up the courage to stay where they belong.

      A testicle that remains inside the body (a condition called cryptorchidism) won’t function properly because the temperature is too warm. A boy who has this problem may also be embarrassed by his appearance. For these reasons, medical intervention is usually called for, which may be a type of hormonal therapy but more likely will involve surgery. This condition also puts men at a higher risk of testicular cancer.

      Manufacturing hormones

      In addition to the testicles’ vital role in the continuation of the species (which I discuss in the following “Producing sperm” section), men require functioning testicles for the hormones they produce, most importantly testosterone. Testosterone is called the “male hormone,” and that name truly fits. If a boy is born without testosterone, his scrotum forms as the outer lips of a vagina and his penis as something akin to a clitoris.

      Producing sperm

Despite the fact that a variety of contraceptive methods have allowed people to disconnect sexual intercourse from reproduction, the main purpose of having sex, from an evolutionary point of view, is still to make babies. But, although the penis is required to penetrate the woman’s vagina for the best chance at success, the man needs seeds to place within her to accomplish this important task. These seeds, called spermatozoa (or more often by their nickname, sperm), are manufactured in the testicles.

      Sperm are rather amazing little creatures. They are the only parts of the body that do their work outside of it. You see, sperm don’t survive well at high temperatures, particularly the temperature inside our bodies. This is why the testicles lie outside the body where they can be cooled by the soft summer breezes (at least for those of you who favor kilts or loincloths).

For the sperm to be successful at their task of making babies, they have to overcome many obstacles after making a long journey. You may well recognize their final shape — an oval head with a long tail that helps to propel them along — but sperm don’t start out that way (see Figure 2-4).

      Illustration by Kathryn Born

      FIGURE 2-4: The sperm: independent, a good swimmer, and a heckuva guy.

      From humble beginnings

      Early in their life cycles, sperm are called germ cells. (In this case, I think most people would have preferred a nice, long Latin name; but rest assured, these cells have nothing to do with what we commonly associate the word germ.)

      Germ cells are produced in the seminiferous tubules, which are long, spaghetti-like tubes that are connected to each other, packed into a tight ball, and surrounded by a tough membrane. This package is called — drumroll, please — a testicle. (Between these tubes are cells that produce the male hormone testosterone.) As the germ cells travel along the tubes, slowly but surely they turn into sperm.

Their metamorphosis complete, the sperm leave the testicle and head for the epididymis on their way to the vas deferens. (Now’s the time to look at Figure 2-5, and be amazed that sperm can find their way without any help from GPS.)

      Illustration by Kathryn Born

      FIGURE 2-5: The sperm leave the testicle and head for the epididymis and vas deferens.

      Meiosis: Small division

      Now that you’ve had a chance to look at the diagram and can picture in your mind’s eye the journey that the spermatozoa take, I have to tell you about one more important transformation that they make.

      

All of our cells have the complete code of genetic material, called DNA (the long term is deoxyribonucleic acid, but DNA is much easier to say), unique to each individual. But while the germ cells start out with all of this DNA, along the way they undergo a process called meiosis (pronounced “my-oh-sis”). Here are some of the important effects of meiosis:

       When a germ cell undergoes meiosis, it forms two new cells, each of them having only half of the DNA code: 23 bits of genetic material (called chromosomes) instead of the normal 46.

       When a sperm teams up with a female egg, which also only has 23 chromosomes, their genetic materials intertwine, and the resulting baby ends up with a package containing a total of 46 chromosomes that is a mixture of both the mother’s and the father’s genetic material.

       When the male germ cells divide, the sex chromosomes divide also. The male has one X and one Y chromosome; the female has two X chromosomes.

       Whether the sperm that reaches the egg first has an X (female) or Y (male) chromosome determines whether the baby will be a girl or a boy.

      

All of you macho men out there may appreciate knowing one more thing about a spermatozoon (a single, fully developed sperm; spermatozoa is the plural): Not only can sperm move around on their own outside the body, but they’re also fully armed, like little guided missiles. Over the head of the sperm lies the acrosome, which is full of enzymes that help the sperm penetrate an egg if it should be so lucky as to meet one on its journey.

      When they’re ready, the sperm leave the testes (another name for testicles) and enter the epididymis, which is a series of tiny tubes that lie on top of the testes. (For those of you into amazing statistics: If unfurled, these tubes would reach 60 feet in length.) During their journey through the epididymis, sperm learn to swim. They enter the epididymis with useless tails and leave it as little speed demons. (See the advice that practice makes perfect lies at the very heart of human life.)

      Vas deferens

      If you go back to Figure 2-5, you see that the sperm’s next stop on their voyage is the vas deferens, a tube that ejects the sperm into the urethra, through which semen and urine pass. In the urethra, the sperm

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