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  At the end of the sixteenth century scientists brought new tools to the question of the source of semen’s power. In 1590, an early microscope was crafted by eyeglass makers in the Netherlands; within thirty-five years, Galileo Galilei had built his compound microscope, which he called his ‘little eye’. Then, in 1670s Delft, a Dutch cloth merchant and surveyor named Antonie Leeuwenhoek turned his hand to lens grinding. Leeuwenhoek handcrafted around three hundred lenses, improving the technology from the poorer models that were available, though at first sight his efforts are barely recognizable today as microscopes. Crafted in brass or silver, he made them in a variety of tiny shapes; some looked like the flat end of an oar, others like an elegant handheld fan, a few like a toilet plunger. Leeuwenhoek was more than a tinkerer, though, and used his microscopes to make a number of discoveries: of single-celled organisms, now called protists, in 1674, and of bacteria, two years later. He was also perhaps the first person to use these novel instruments to observe semen up close.

  At first, it seems he was less than keen about putting semen under his microscope, or studying anything to do with sex, for that matter. This changed in 1677, when Johan Ham, a medical student, called on Leeuwenhoek at his home and presented him with a sample of semen that had been extracted from a patient with gonorrhoea. Ham thought he had seen small animals with tails writhing around in the fluid, and wanted confirmation. The claim captured Leeuwenhoek’s interest. He began observing his own semen – acquired, he stressed, ‘not by sinfully defiling’, but from natural conjugal coitus. Through his crude microscopes he confirmed that there were ‘a multitude of animalcules, less than a millionth the size of a coarse grain of sand and with thin, undulating transparent tails’. Since he had been studying his own semen, the animals were unlikely to have been parasites or linked to gonorrhoea – in Leeuwenhoek’s scientific opinion.

  Nevertheless, based on his reports, the tiny, tadpole-like creatures came to be known as ‘spermatic worms’, from sperma, Greek for ‘seed’. In 1700, they were included in a book on human parasitology, An Account of the Breeding of Worms in Human Bodies, by Nicolas Andry, an influential proponent of the idea that life was generated only by sperm. In 1820, when the modern name spermatozoa – adding the Greek zoa for ‘living being’ – was coined, sperm were still considered to be a sort of parasite. (Around that time, Richard Owen, Charles Darwin’s contemporary and bête noire, even classified sperm into the group of parasitic worms called Entozoa.) It is understandable that what appeared to be a moving, living being should have been taken to be a symbiotic animal that infected the life-infused semen of males, but not the reproductive fluids of females.

  Having seen sperm first-hand, and being unable to detect the presence of anything similar in women, Leeuwenhoek himself began to suspect that the female ovaries were ‘useless ornaments’. He noted that male rabbits that were grey only ever produced other grey rabbits – evidence that semen provided the sole contribution to the creation of offspring. He considered it ‘proof enabling me to maintain that the foetus proceeds only from the male… and that the female only serves to feed and develop it’. Leeuwenhoek further claimed that his semen sported complex anatomical structures – nerves, arteries, veins – though no one else was able to observe them. He made a point of emphasizing these features in his drawings, noting that in semen ‘there may be as many parts as in the human body itself’.

  In 1694, the Dutch mathematician and physicist Niklaas Hartsoeker built on Leeuwenhoek’s work to describe what the preformed animalcules looked like. Hartsoeker, who worked with rooster sperm, claimed that it was he who in fact had first discovered the animalcules in sperm, not Leeuwenhoek. In any case, it was Hartsoeker who first made the animalcules tangible to those who had not seen them with their own eyes. In his Essai de dioptrique, on optical instruments, he published a drawing of the homunculi, or little people, who inhabited each sperm. Hartsoeker described the egg as ‘no more than what is called the placenta’, once again defining the female’s function as nothing more than nurturing a foetus that had been formed from semen, now sperm, alone. But then, Hartsoeker hadn’t actually seen the animalcules with his own eyes; he had simply imagined that they might look like tiny, perfectly formed children, complete in every detail. As the head of one sperm, he drew a child curled up in a foetal position; in the other two sperm, the heads are children sprawled out, seemingly asleep or in a state of suspended animation. Each sperm’s tail dangles from the children’s pates like a Victorian man’s nightcap. In his musings, Hartsoeker went on to suppose, correctly, that a foetus growing in a womb would require the means for becoming physically attached, in some way, to its mother. This, he proposed, was the purpose of the tail of the sperm, which would subsequently develop into the umbilical cord.

  Hartsoeker’s drawings represented no more than fantastical speculation, but five years later, in 1699, a French aristocrat and astronomer named François de Plantades reported that he had seen exactly what Hartsoeker had predicted. Peering through his microscope, Plantades said he had spotted miniature human forms, tucked inside the heads of each sperm. Perhaps for reasons of professional etiquette (he served as secretary of the Montpellier Academy of Sciences), he published his findings under the pseudonym of Dalenpatius, with his paper appearing simultaneously in London, Edinburgh, and Amsterdam. Dalenpatius’s claim, however, was nothing more than a hoax, an attempt by Plantades to ridicule those who believed in preformed, make-your-own humans and microscopic animalcules. If his goal was to bring the whole field into disrepute, he was grossly unsuccessful. The existence of strange and mysterious creatures in sperm gained new credibility, and the little sperm people became entrenched in popular belief for the next one hundred years.

  In this way, even though scientists now had the tools to investigate the body and no longer had to rely on intuition, many swore they saw things that simply did not exist – and would point to the microscope as their proof. And so reproductive science continued to remain faithful to the ideas promulgated by Aristotle and Galen.

  These ideas were repeated in the widely circulated Aristotle’s Masterpiece, a compendium of medieval medicine and folklore thought to have been written around 1680. (It is also known as The Works of Aristotle, though it was certainly not penned by the philosopher.) Aristotle’s Masterpiece includes some excerpts from his work, as well as of the writings of Galen and the tenth-century Islamic physician Ibn Sina, who himself wrote a commentary on Aristotle’s findings. The book includes descriptions of midwifery, female reproductive organs, and all things related to sex and embryos. Because of its sexual content, it was considered pornographic, so much so that it was banned – and remained banned in the United Kingdom until 1960. In the United States, however, Aristotle’s Masterpiece was more accepted. Until the middle of the nineteenth century, it was the most commonly read medical text – despite the arrival of the new microscopes, dissection tables, and complex experimentation, which completely contradicted the book’s depictions of the workings of reproduction.

  For nearly two millennia, sperm reigned supreme. Then, it was discovered that mammals also had eggs.

  The year was 1827, and a German scientist, Karl Ernst von Baer, was investigating the reproductive tract of a bitch. It had of course long been obvious that birds and reptiles had eggs; these were in plain sight. By the seventeenth century, it was suspected that mammals might have them, too, although no one had been able to find one. Leeuwenhoek had searched for a mammalian egg with his increasingly sophisticated microscopes, but he had thrown off the hunt as a lost cause. Using a better microscope, however, von Baer had been able to distinguish a yellowish-white, point-like object within some structures, called follicles, that he had taken from a dog’s ovaries.

  Von Baer was curious, so he sliced open a follicle, used the tip of his knife to remove the pin-prick object, and placed it under his microscope. ‘It is truly wonderful and surprising to be able to demonstrate to the eye, by so simple a procedure, a thing that ha
s been sought so persistently and discussed ad nauseum in every textbook of physiology as insoluble’, he later wrote of his momentous discovery. He was ‘utterly astonished’ to see the egg with his own eyes ‘and so clearly that a blind man could hardly deny it’. But blind men there had been aplenty – including Leeuwenhoek.

  To Leeuwenhoek, eggs existed so that the preformed embryos in sperm could be implanted in them. His stubbornness is all the more surprising when you consider that in addition to the discovery of sperm, the Dutchman is credited with the discovery of parthenogenesis, the development of the egg into a new individual being without fertilization by sperm. If you weren’t too sure that eggs existed, as Leeuwenhoek said he wasn’t, you might say that this process amounts to a female bearing offspring with no lasting input from a male – the equivalent of a virgin birth. And Leeuwenhoek was the first scientist to notice that female aphids had virgin births all the time.

  An avid gardener, in the summer of 1695 he became somewhat concerned that the leaves of his gooseberry, cherry, and peach trees were damaged. At first he thought the mutilations were the work of ravenous ants, but on closer inspection, he spied aphids. Leeuwenhoek did with the aphids what he did best: he pulled out a microscope and, as had become his custom, he searched for the eggs of this new species. He found none. He then dissected what he guessed were the females. He found no eggs in them either. But he did find miniature, preformed aphids. The first specimen he dissected contained four young, and he removed as many as sixty from another.

  This should have put an end to the idea that male semen, or sperm, was the sole instigator of new life. But there again, Leeuwenhoek had chanced upon an organism in which reproduction is by no means straightforward. The sexual tactics employed by female aphids are tricky and complex. Two hundred million years ago, the insects evolved a reproductive strategy that allows them to practise reproduction by parthenogenesis – in cycles. This means that female aphids do have eggs, and both the fertilized and unfertilized eggs of a female are capable of forming embryos. The small aphids that Leeuwenhoek observed when he cut open his female – those born live as a result of parthenogenesis – were exclusively female. What is more, a single female generated by parthenogenesis may contain three generations within her body: the numerous embryos of her unborn daughters and, within them, her granddaughters-to-be in the early stages of development. For aphids this amounts to a brilliant strategy for rapidly producing an immense population; a virgin female can, in theory, produce billions of offspring in a lifespan of roughly one month. Here was a stack of Russian dolls, miniature yet fully formed creatures in ever smaller packages, just waiting to be born – a perfect preformed embryo, but from a female.

  Despite this finding, and von Baer’s production of the elusive mammalian egg from a dog, the egg continued to be considered the lesser element of reproduction into the Victorian age. In 1849, Richard Owen, who had classified spermatozoa as parasites, delivered a talk at the Royal College of Surgeons entitled ‘On Parthenogenesis, or The Successive Production of Procreating Individuals from a Single Ovum (Egg)’. Owen had coined the word ‘parthenogenesis’, yet he could not extricate from his mind the influence of sperm over the process. Instead of the potential of eggs to self-reproduce, his lecture expounded on the virtue of sperm. For him, a ‘virgin birth’ could only ever follow an original fertilization event – and fertilization required sperm. What he called ‘spermatic virtue’ was a power contained in sperm that could be divided equally among countless offspring. He told his audience that the development of an embryo by parthenogenesis differed from a normal fertilization involving sperm ‘only in... non-essential particulars’, by which he meant that the power of sperm was the absolute requirement.

  In the late nineteenth century, Owen would come under fire for this explanation. His critics were formidable – among them Darwin and Darwin’s ‘bulldog’ supporter, Thomas Henry Huxley. Huxley was professor of general natural history at London’s Imperial College and had contributed substantial knowledge to the growing field of comparative anatomy and palaeontology. He levelled great criticism at Owen’s science and methods; in return, Owen published cloaked insults about Huxley’s own work. It’s fair to say that their earlier friendship had dissolved by this time. A colleague of Huxley’s even advised him to shoot Owen in a duel. The two scientists had running arguments on anatomy, aphids, and parthenogenesis. Huxley particularly attacked Owen’s references to a non-descript spermatic virtue or ‘force’ that could be retained through generations of aphid females, calling such speculations ‘ignorance writ large’. For his part, Darwin egged on Huxley to challenge Owen on this point.

  Meanwhile, in Germany another distinguished professor of comparative anatomy, Karl Ernst von Siebold, also ridiculed the belief in all-powerful sperm. Siebold did not respond by producing more speculation, but by performing exhaustive experiments to investigate ‘true parthenogenesis’. This was, in contrast to Owen’s definition, the development of an egg that was perfectly capable of being fertilized by sperm but which had not been. For his test subjects, Siebold turned to aphids, bees, and moths. He knew the conviction that eggs must be exposed to spermatozoa before they can develop was very deeply rooted; he himself had once been a strong opponent of the existence of parthenogenesis.

  In 1857, after years of study, Siebold published his findings on bees and Psyche and Solenobia moths; in a nod to Richard Owen, he entitled his text On a True Parthenogenesis. In his exacting observations, Siebold had noted that the unfertilized eggs of his moths produced female offspring, but that queen bees produced male drones through parthenogenesis and female offspring from eggs fertilized by sperm. Contrary to Owen’s definition, it was clear that new organisms could develop solely from eggs. Siebold also uncovered that not only can eggs develop into fully formed animals quite without any fertilization event but also that parthenogenesis 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 miner
als, 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.