The Seeds of Life Page 8

  For starters, semen and menstrual blood both seemed to play some part in the sex story, though it was by no means clear just what. (Detectives would have dubbed the human counterparts of such shady characters “persons of interest.”) Aristotle rattled off points to ponder. Young boys did not produce semen, which appeared on the scene only when they reached physical and sexual maturity. Similarly, young girls did not menstruate, and neither did old women; menstruation overlapped precisely with a woman’s child-bearing years.

  Now Aristotle hammered home his argument. First, blood played a vital role in the body. Ordinary blood provided nutrition; menstrual blood must therefore fulfill some crucial mission of its own. Second, the embryo grew within the mother’s body and was a tangible, physical structure that had not been present before the start of the pregnancy. Third, a pregnant woman no longer discharged the blood she had produced each month before her pregnancy.

  The conclusion practically declared itself. Something appeared in pregnancy, and something disappeared. It should be plain to all, Aristotle announced, that the embryo was formed from the mother’s menstrual blood. The semen shaped this raw material as a sculptor fashions clay. (As a bonus, the theory also explained the birth of females. Sometimes, Aristotle explained, “owing to youth, old age, or some similar cause,” a man was too weak to produce proper, vital semen. If he didn’t have enough oomph to beget sons, he would have to settle for daughters.)

  FOR US, THIS FOCUS ON MENSTRUATION SEEMS PECULIAR. BUT OUR picture derives from the notion of menstruation as the flushing out of an unfertilized egg. The Greeks, who had never seen mammalian eggs, framed things quite differently.

  Men and women had an inherent “vital heat,” they believed, and it was this property of living organisms that set them apart from rocks and pots and other inert lumps. Though hard to define, heat was akin to vitality or soul or vigor. In modern terms, it corresponded more or less to metabolism. Aristotle, who favored homey images, talked of how the body nourished itself by means of a sort of “cooking.”

  Heat was a good thing, and men had more of it than women. “It rose naturally toward the heavens and towards the brain,” writes the historian Merry Wiesner, “which explained why men, being hot and dry, were more rational and creative; women, being cold and wet, were more like the earth.”* This was a far-reaching theory. It explained why women menstruated and men did not (men “burned up” their excess blood), and why men went bald and women did not (men “burned up” their hair), and why women had wide hips and men did not (women did not have enough heat to propel their flesh upward).

  When it came to sex and reproduction, “cooking” transformed blood into semen by a process of purification and refinement. In women’s colder bodies, cooking never got properly underway and therefore achieved a less impressive transformation, merely converting ordinary blood into menstrual blood. Semen and menstrual blood were analogous, but semen was the high-grade product, menstrual blood its inferior counterpart. Semen was rare and valuable, a nectar made in tiny quantities. Menstrual blood was raw and unfinished, a crude and overabundant home brew. “In a weaker organism,” Aristotle explained, “there will inevitably be a greater flow of less fully cooked blood.”

  This half-baked theory won wide support from the era of the Greeks to the time of Harvey and the king’s deer. Aristotle believed he had dealt a death blow to the idea of female semen, which had floated around long before Galen spelled out the details. Since women were cold, it made sense that they produced menstrual blood, which was raw, rather than semen, which was cooked. How could anyone believe that women produced both?

  To imagine such a thing, Aristotle scoffed, would be to claim that “woman is at one and the same time of hot and cold temperament, which is the height of absurdity.” One might as well claim that a stone could sit atop the water, because it was so light, and simultaneously sink beneath the waves, because it was so heavy.

  The way that conception worked, as Aristotle set it out, was that both semen and menstrual blood played vital roles, but it was semen alone that shaped the new life. Every handmade item you might name—a loaf of bread, a clay jug, a stone house, a wooden chair—involved a craftsman transforming raw material. For living creatures, Aristotle proclaimed, exactly the same pattern held: a creative, shaping force took a bit of humdrum stuff and transformed it.

  One sex performed magic; the other provided supplies. Three guesses.

  PART TWO

  THE SEARCH FOR THE EGG

  “How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?”

  —ARTHUR CONAN DOYLE, The Sign of the Four

  SEVEN

  MISSING: ONE UNIVERSE (REWARD TO FINDER)

  BOTH OF THE MAIN ATTEMPTS AT EXPLAINING THE MYSTERY OF sex and babies—Galen’s one-sex, two-semens model and Aristotle’s semen-and-menstrual-blood variation—had endured from before the birth of Christ to the dawn of the modern age. Both seemed a mix of the plausible and the bizarre (men and women the same? embryos built of menstrual blood?). Neither side claimed its case was airtight. Aristotle, who was a brilliant and tireless student of the animal kingdom, knew perfectly well that many female animals did not menstruate. How could it be, then, that their embryos were formed of menstrual blood? The answer he came up with was that these animals did produce various secretions, and those fluids played a role akin to menstrual blood.

  The argument smacks of special pleading. But even if we put animals aside and look only at humans, comparing semen and menstrual blood was a stretch. What was the similarity, really, between a once-a-month fluid and one that appeared each time sex took place? As we have seen, the ground that Galen stood on was just as shaky. What was the true resemblance between a uterus and a scrotum?

  It was impatient, dyspeptic William Harvey who proposed to break the impasse. He would do it not by talking but by experimenting. Harvey had a flair for designing experiments to resolve questions that had seemed doomed never to move beyond insult and guesswork. Early in his study of the heart, for instance, he had been stymied by just how hard it was to see what was happening. He had looked inside the chest of a living dog and seen its heart contract and expand (any dog lover would grow queasy at the details of these hideous experiments), but the sheer speed of the action made it impossible to understand the big picture.

  Then it had dawned on Harvey to look at cold-blooded creatures like frogs, snakes, snails, and shrimp. He found with a thrill—the discovery presumably set his heart racing—that their hearts beat ever so slowly. He dissected countless amphibians and reptiles. Centuries before the invention of the slow-motion camera, Harvey had devised a way to slow down the beating heart so that he could study it in detail.

  This strategy of approaching a problem by way of an easier example would become one of Harvey’s favorites. It had brought him ridicule—What do lowly snakes have to do with noble humans?—but in the end it provided insights into the heart that had eluded all his predecessors. Now, with the mysteries of reproduction on his mind, Harvey took advantage of his friendship with the king. He set out to explore the royal menageries.

  The ostrich, the world’s largest bird, caught his eye at once. The bird’s style of mating was as striking as its size. Harvey looked on, agog. “I myself have seen a hen ostrich, when her keeper gently stroked her back with the intent to arouse her desire, throw herself on the ground… and disclose and stretch out her vulva,” he wrote. “When the cock bird saw this, being instantly enflamed with desire, he mounted her, and with one foot on the ground and the other pressing on her back as she lay, accomplished his purpose with an exceedingly large and vibrant yard [i.e., penis] that you might have taken for a [cow’s] tongue. All this went on with much muttering and noise on both their parts, stretching out and pulling back their heads and many other signs of rejoicing.”

  When captive ostriches happened to die, Harvey hurried to dissect them, apparently on the theory that the h
uge birds would serve as a kind of large-print primer on the secrets of reproductive anatomy. The appeal of this strategy was hard to deny—an adult ostrich and an adult human are roughly the same size, but an ostrich egg is larger than any other animal’s, about five inches in diameter. (The eggs of all mammals, from dachshunds to giraffes to humans, are virtually the same size.) Perhaps what was hidden in the human being would proclaim itself in the giant bird.

  But birds turned out to be an uncertain guide to human anatomy. Though male swans, ducks, and ostriches have penises, for instance, the males in many smaller species do not.* At the end of all his studies, Harvey found himself well-informed on bird anatomy but unsure what to make of his newfound knowledge. “In a black drake I once saw a penis of such length that, after coition, a hen pursued it as it trailed along the ground and eagerly pecked at it, believing it, I am sure, to have been a worm, and this made the drake retract it more quickly than is his custom.”

  Earlier observers had found themselves not merely confused by the complexities of sex in birds but positively outraged. In 1474 a huge crowd had gathered in Basel, Switzerland, to see a rooster burned at the stake. The bird’s sin was that, even though he was to all appearances a bold, strutting, cock-a-doodling male, “he” had laid eggs. This was contrary to nature and dangerous besides—the eggs of such hybrid creatures were well-known to hatch into half-bird, half-serpent creatures whose mere glance could kill—and execution was the only fitting response.

  Oddly, Harvey had made a similar mistake about the sex of a bird. Harvey’s wife had a beloved pet parrot who sang on command and sat in her lap to have his head scratched. (This meager fact is virtually all we know of Elizabeth Harvey. It is tempting to imagine her patiently reciting, “Who’s a pretty bird?” while her husband sliced up cats and dogs and mice in the next room.) After many years, Harvey recalled, the bird “fell sick, and by being seized with repeated attacks of convulsions, died, to our great sorrow, in its mistress’s lap, where it had so often loved to lie.” Harvey knew that only male parrots speak and sing. He dissected his pet to see what it had died of and found, to his amazement, “an almost complete egg in its oviduct.”

  Armed with countless but confusing bits of information from the royal menageries, Harvey needed to find a better way to demonstrate to the world that mammals did have eggs, as he had long believed. This was the point of his deer-hunting excursions with the king.

  The king’s deer, which mated and conceived on the same predictable schedule year after year, seemed ideal research subjects. A devoted admirer of Aristotle, Harvey felt certain that he would confirm his master’s teachings: when he examined the bodies of newly pregnant deer he would find within the uterus a small, egg-shaped embryo fashioned from the female’s menstrual blood by the male’s semen.*

  But he didn’t. He found nothing at all. He looked again. Still, he found nothing. He looked at pregnant dogs and found nothing. At rabbits, nothing. Stymied in his search for semen, menstrual blood, or embryo—for any clue he could touch—Harvey drew the only possible conclusion. Females conceived “as by Contagion.”

  In a world that had seen whole countries devastated by plague, this was perhaps a natural thought. Harvey was not invoking magic. He pointed out that “epidemic, contagious, and pestilential diseases scatter their seeds and are propagated to a distance through the air.” If plague could scythe through whole cities, passing from one person to another invisibly and unaccountably—without a sting or bite or wound—why could not semen do its fertilizing work at a distance, without physical contact?

  Semen evidently exerted its influence from afar, as a conductor’s baton guides an orchestra. Its physical absence in the female’s body testified to its power. This potent fluid contained something, Harvey wrote, “analogous to the essence of the stars.”

  Harvey cited other analogies. Magnets were much in vogue in the 1600s, because they produced a force that was both undeniable and inexplicable. Magnets could draw iron filings across empty space; why should semen, too, not act at a distance?

  Most significantly, perhaps, Harvey compared the uterus and the brain. The brain secreted thoughts; an idea was often described as a “conception” of the brain. It was telling, Harvey claimed, that we used the same word when we talked about the “conception” of new life. An embryo was a “conception” of the uterus. Neither “conception” was a physical object. No wonder, then, that no matter how closely you examined a newly opened uterus, you saw nothing.

  TEXTBOOK ACCOUNTS OF SCIENCE SAY THAT YOU TEST A THEORY by performing an experiment. If the result contradicts the theory, you throw the theory away and think up a new one. But it seldom works that way. Harvey sliced open his deer in the full expectation that he would see a tiny embryo that he could flick into the sunlight with the tip of his knife. When he didn’t find it, he scratched his head in bewilderment, but he stuck to his theory. It had to be right. Sooner or later, he knew, he would see how.

  Modern scientists display the same reluctance to abandon trusted theories, in even the most daunting circumstances. When the New York Times reported that “Ninety Percent of the Universe Found ‘Missing’ by Astronomer,” the story made page 1, but scientists blithely carried on with business as usual. The Times noted merely that the great figure in the field, a Princeton physicist named Martin Schwarzschild, “hopes the missing matter in the universe can be located.”

  When Harvey peered into his deer, perhaps the prudent conclusion would have been, “I cannot see anything and therefore I cannot conclude anything.” But one of Harvey’s greatest coups had come from taking precisely the opposite line. In tracing blood’s voyage around the body, he had lost sight of his quarry: he knew that the heart pumps blood into the arteries, which nourish the body’s tissues, and he knew that eventually blood returns to the veins, which carry it back to the heart. But, limited by the technology of his era, he could not see how blood made its way from the arteries to the veins.

  He deduced—correctly and brilliantly—the existence of a network of blood vessels too tiny to make out with the naked eye. They are now called capillaries, and they were first seen only after Harvey’s death. “I cannot see anything,” he had reasoned, “and I know exactly what to conclude from that.”

  When it came to sex and conception, too, Harvey carried on undaunted by his quarry’s vanishing. (We now understand how Harvey’s deer experiments misled him. Harvey could never have known it, but deer might as well have been designed to hoodwink a scientist. Deer embryos don’t have the familiar round shape that Harvey expected from dissecting rabbits and dogs; they are long, slender, strand-like, and easy to miss even when you know what to look for.

  Worse, there is typically only one embryo per doe, and that one hardly begins to grow until ten days after conception. Harvey did notice “mucous filaments like spiders’ threads,” but he did not give them any special notice. If the king’s hunting parties had been designed to pursue rabbits rather than deer [and if Harvey had waited a bit before cutting open the pregnant rabbits], the trophies in the royal lodge would have had an odd look, but Harvey might have found what he longed to see.)

  In his old age, Harvey published his second great work, Disputations Touching the Generation of Animals. He was seventy-three, famous but in bad health and in bad spirits. Tortured by gout, he tried to ease the pain by submerging his legs in tubs of ice water “till he was almost dead with cold,” in the words of an acquaintance. Then he would stumble to the stove as fast as he could, to thaw out.

  These were harsh times. With his beloved king executed and many of his own research papers destroyed during the civil war, Harvey had opted to retreat from the world. (By one account he tried to kill himself by swallowing opium.) A friend persuaded him that his decades of work should not go unrevealed, and Harvey handed over the manuscript of the Generation of Animals.

  This took considerable coaxing, for Harvey still grew indignant when he recalled the scorn that had greeted his book on the heart. Tha
t criticism had long since turned to adulation, but Harvey had not mellowed. Most infuriating of all, his admirers had misinterpreted the message of On Circulation and placed Harvey at the head of an army he despised.

  The world hailed him as the man who had proved that the body was a machine. This was nonsense, Harvey thundered. He wanted nothing to do with those who would banish God and spirit from the world in favor of soulless pipes and valves. Those who would reduce the living world to mere mechanism were “shit-breeches.”

  Harvey had ventured out ahead of the evidence when he proposed that the body has capillaries, and in the new book he did so again. His claim, destined to become famous, was that all life comes from an egg. This was a sweeping assertion, embracing egg-laying animals like birds and live-bearing mammals like dogs, cats, and humans. “All animals whatsoever… nay, man himself, are all engendered from an egg,” Harvey wrote.

  In a poem published as the dedication to the book, an admirer of Harvey’s put the claim even more boldly. In the ancient battle between analogies—Were women like men, or were women like other female animals?—the poem left no doubt about Harvey’s view:

  Both the hen and housewife are so matched,

  That her son born, is only her son hatched.

  A drawing on the title page depicted Zeus on his throne, holding open an immense egg, one half in each hand. A horde of living creatures spilled out: a bird, a baby, a spider, a butterfly, a fish, even (despite its obstinately missing embryo) one of the king’s deer. Zeus’s egg bore a Latin motto, forever after associated with Harvey. Ex ovo omnia, it read. Everything comes from the egg.