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was by no means an exceptional occurrence, something peculiar to aphids. He made a point of countering the idea that ‘development of the eggs can only take place under the influence of the male semen’. This age-old concept, he wrote, ‘has suffered an unexpected blow’. Rather than being a result of some undefined force of questionable existence, parthenogenesis was an independent, fixed, orderly event.

But if eggs could develop on their own, as Siebold had proved, then what was the point of the male? Based on Siebold’s work, Darwin made a remarkable conjecture: ‘I have often speculated for amusement on the subject, but quite fruitlessly,’ he wrote to his friend Huxley, ‘But the other day I came to the conclusion that some day we shall have cases of young being produced from spermatozoa ... without [an egg].’ Darwin had a point: if eggs could independently generate life, why couldn’t sperm do it, too? And if not little people, what was inside the sperm, and how were these seemingly living creatures made?

In 1905, Jacques Loeb provided an answer. Loeb, a physiologist working in Germany, was busy trying to force unfertilized eggs to develop into embryos. Using alkaline or acid solutions, potassium, and salt, even ox blood and cane sugar, he triggered development in the unfertilized eggs of sea urchin, starfish, marine molluscs, and other creatures. For the first time in history, someone had managed to create new life in the laboratory with no sperm at all.

In working out what to substitute for sperm, Loeb realized he needed to find something that must have two effects on the egg. ‘In the first place’, he wrote, to ‘cause... its development’ and in the second to ‘transmit... the paternal characters to the developing embryo’. For the marine species with which he was experimenting, the ability to cause the embryo to develop was enough. Baby sea urchins born in his lab would need no fathers from which to acquire paternal characteristics. The same could not be said if the subject were not sea urchins but humans.

The fluid praised as the essence of life by Aristotle and Galen (and the innumerable others who came before and after) is indeed remarkable. Human semen is a rich cocktail, a combination of sugars, salts, enzymes, vitamins, and minerals, including such truly essential ingredients as fructose, sorbitol, inositol, phosphorus, zinc, magnesium, calcium, potassium, ascorbic acid (vitamin C), and cobalamin (vitamin B12). As the ancient thinkers suspected, it is also the medium through which a father provides his set of instructions for making offspring. But unknown to these early natural philosophers, some part of semen – about five percent of what a man ejaculates – contains fifty million to two hundred million sperm. These cells are highly specialized, built to travel up to four millimetres a minute and to release chemicals that can target and penetrate the egg.

The creation of sperm begins inside the testes of a pubescent boy, when the solid cords that had transected these glands throughout his childhood begin opening up into tubes. The process carves a space at the cord’s centre through which fluids will eventually be able to pass. These tubes will become contorted and so numerous and fine that in an adult male testicle, their collective length will measure as much as 350 metres, or more than one thousand feet. They will also become home to the stem cells that become sperm, called spermatogonial stem cells, or SSC. Stem cells are by definition immature, in that they are somewhat undecided as to their identity and 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. Some sperm 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