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therefore retain the ability to become something different – something
more definitive, more specialized. At the start of a wonderfully efficient production line, these rounded cells are the first widgets in the manufacture of mature, tadpole-like sperm and,
ultimately, are the basis of male fertility. Each sperm is moulded out of the contents of these stem cells, then conveyed into holding areas, a bit like reservoirs, which line the outer layers of
the fine tubules that now populate the testes. Driven by the male sexual hormone, testosterone, developing sperm will move in waves of output along the belts of these tubes, which eventually spiral
like a corkscrew, in towards the space at the tube’s centre. In the space of roughly sixty-four days, they will be transformed from round, nondescript cells to fully fledged sperm with heads
and tails; from being tucked away in inventory to positioning themselves in readiness for consumption – ejaculation.
    If that ejaculation happens in the context of unprotected sex, it will only be possible for one out of the many millions of sperm to make it successfully into an egg. If one penetrates the egg,
the egg will harden to prevent another from entering. That is, if any succeed in entering at all. Achieving fertilization is a formidable task, and requires sperm that are fit for purpose. In the
process of making sperm from stem cells, many defects occur. Their heads may be too large or too small, tapering or shapeless. They may even have two heads instead of one. Somesperm are made with bent tails, or tails that are too thin, too long, or too short, broken, coiled, or altogether missing. Some sperm have been found with a combination of defects.
Sperm with tail abnormalities will have little chance of swimming well enough to get anywhere near an egg. Those with head defects may be carrying damaged DNA, or an abnormal amount of DNA. They
may also be unable to use their head to penetrate an egg properly, even if they did get close enough.
    The environment in which sperm are made may have a lot to do with how well they are formed, or the quality of their genetic material. Low levels of testosterone, and of the minerals zinc and
selenium, seem to be bad for sperm and male fertility. Lifestyle and other factors of biology can also affect the integrity of the genes carried by sperm: exposure to radiation, heat, cigarette
smoke, airborne pollutants, or chemotherapy drugs; sexually transmitted infections; ageing; a high body mass index; and medical conditions such as insulin-dependent diabetes – all can degrade
the quality of DNA.
    But even for perfectly formed sperm, there are many obstacles to overcome. In order to reach the egg, they must survive the acidic atmosphere of the vagina and avoid getting trapped in the
sticky mucus in the cervix, the gateway to the womb. If they make it through those hurdles, they will then have to navigate the narrow entrance into the cervix, dodge cells of the immune system
that will try to target and destroy them, swim upwards against the current, and escape a final molecular process that will screen and eliminate the vast majority of whichever sperm have survived.
Sperm will do all that for only one reason. On a scale that is about one thousand times smaller than a mustard seed, the head of the sperm carries the genetic instructions to start making a baby
– an essential ingredient of sexual reproduction. In fact, somewhat as Aristotle suspected two millennia ago, what the semen delivers into the egg will contribute tothe
form of a resulting child – its looks and its general genetic make-up – but not the ‘matter’. This is because, if and when a sperm makes contact with an egg, only its head
penetrates, so that it can release its DNA-containing package into the awaiting receptacle. This DNA is its only contribution.
    In contrast, at one hundred times larger than a sperm, the egg is mostly composed of a large amount of cytoplasm, or