Countless hours of observation soon made him expert on what he called “the amours” of the frog. (He also studied what one historian called “the nuptials of the newt.”) The male frog clasps the female from the back, she exudes a stream of eggs into the water, he releases his semen on them, and the partners swim apart. (Mating males clasp their partners so tenaciously that Spallanzani found it almost impossible to pry them away. Not even amputation—first of one limb, then of two, and then of the male’s head!—dampened their enthusiasm.)
This certainly looked like external fertilization, contrary to Linnaeus, but Spallanzani was a man who dotted every i and then went back and polished the dots. It was hard to be sure just what was happening in all that water, especially with the female “darting backward and forward” and the male “throwing himself into strange contortions,” and both of them croaking away all the while. Spallanzani put pairs of frogs into empty, dry containers, where it was easier to keep watch. Even in this unfamiliar setting, the partners carried on undaunted, the males releasing “a small jet of limpid liquor” upon the eggs. Spallanzani put those eggs into water and kept watch. They grew into tadpoles, which grew into frogs. In frogs, fertilization takes place outside the body. That took care of Linnaeus.
FIGURE 19.1. This statue of Spallanzani stands in the town square in his hometown, Scandiano, in northern Italy. The frog is not wearing pants.
With the preliminary obstacles cleared from his path, Spallanzani marched decisively ahead. But in quest of the biggest prize of the age, the big-game hunter could hardly have looked less imposing. The hero’s weapons were not rifles or spears but needle and thread. Spallanzani had assigned himself a task that sounds like the sort of absurd challenge imposed, in a fairy tale, by a cruel king on a hapless prisoner. He sat at his workbench with cramped fingers and weary eyes, cutting and sewing dozens and dozens of tight-fitting, miniature boxer shorts made of silk. For frogs.
The point of the boxers was to prevent the male’s semen from reaching the female’s eggs. Would the females become pregnant even so, as the “seminal aura” sent out its ghostly waves? Or would the shorts, which were wax-coated as an additional safeguard, serve as a full-body condom?
Spallanzani did not describe the boxers in any detail, and though he was a skilled artist, he made no drawings (it is tempting to picture the shorts as adorned with hearts or even with frogs). “The idea of the breeches, however whimsical and ridiculous it may appear, did not displease me,” he wrote gamely, “and I resolved to put it into practice.”* He wrestled the males into their outfits. Undeterred, they sought out the females with their customary eagerness, Spallanzani wrote, “and performed, as well as they could, the act of generation.”
Then he gathered up the eggs. Half came from females who had mated with boxer-clad males, half from females whose partners had carried on au naturel. Spallanzani peered at the two sets of eggs. Which would grow into tadpoles?
TWENTY
A DROP OF VENOM
SPALLANZANI SOON HAD HIS ANSWER: THE EGGS THAT HAD BEEN doused with semen by naked, unencumbered males developed into frogs; the eggs that semen had not touched did not. Decisive as that seemed, Spallanzani carried out a barrage of follow-ups. He began with the besmirched boxers. He gathered a tiny bit of semen from the cloth and daubed it on some eggs. Those eggs developed into frogs. Untouched eggs did not develop.
This was a milestone in the history of science. Here was an experiment so simple that it could be understood at a glance and that nonetheless, and in one bold coup, resolved an age-old question. Yes, Spallanzani’s demonstration proclaimed, semen and egg are both necessary if fertilization is to occur, and they have to make contact. The “seminal aura” was a myth. In a Museum of Science that featured dioramas of landmark moments, the wing devoted to the 1600s and 1700s might depict Newton conked on the head by an apple (which certainly never happened), Galileo lugging rocks to the top of the Leaning Tower of Pisa (which quite likely never happened), and Spallanzani tightening the waistband of his frogs’ boxers (which absolutely did happen).
But there is a twist in the tale. Considering how odd it is, perhaps it counts as a full-fledged knot. The twist is this: ever since Spallanzani’s day, historians have lauded him as the man who proved beyond doubt that, in the story of sex and conception, sperm and egg are equal players. But Spallanzani never believed that for a minute. Instead, he interpreted his own experiments in a way that later scientists ignored or never even suspected. Convinced that Spallanzani’s marvelous discoveries spoke for themselves, his intellectual descendants credited him with finding truths that he explicitly disavowed. This is a bizarre fate, as if a lifelong pacifist like Gandhi found himself hailed by generations of West Pointers for his brilliance as a military strategist.
What is most striking, to our eyes, is that Spallanzani focused nearly all his attention on semen and almost none of it on the hordes of spermatozoa swimming within that semen. He waved aside those countless swimmers as little more than distractions, probably parasites that happened to live in seminal fluid. Semen, he believed, had some crucial property that sparked eggs to life. Most likely it was chemical. Oddly, he made an analogy to snake venom. By mysterious means, venom had the power to trigger a cascade of dangerous changes in the body; semen was a benign counterpart, a mysterious fluid that triggered life rather than death.
But for so meticulous a scientist as Spallanzani, the first task was to prove beyond the possibility of doubt that semen induced fertilization by direct, physical means rather than by “contagion.” In the experiment where he took semen from the frogs’ underwear and painted it on eggs, Spallanzani had essentially created test-tube frogs. That was a first, and a first that seemed to render any second unnecessary. But the “priest cum scientist,” as one historian described him, was only getting started. He extracted semen from the seminal vesicles of male frogs and painted it on new-laid eggs. Again, tadpoles! (And again, from untouched eggs, nothing.)
Next Spallanzani turned to species where fertilization is internal. In a remarkable display of microsurgery, he extracted semen from male silkworms and spread it on silkworm eggs. Soon, moths! (And no signs of fertilization in untouched eggs.) He tried dogs. He borrowed a spaniel in heat, locked her away from other dogs, and squirted a syringe of semen from a male spaniel inside her. Sixty-two days later, three puppies! (Spallanzani noted without comment that the pups “resembled in color and shape not the bitch only, but the dog also from which the seed had been taken.”)
So semen had to make actual contact with the egg, which seemed to imply that it had some special properties. But perhaps that was too bold a leap. Could other substances also induce fertilization? Persistent to an almost unfathomable degree, Spallanzani gathered up droppers and scalpels and set to work. He began with a bit of blood from a frog’s heart and dabbed it on some eggs, to see if anything happened. Nope. (The rationale was that a beating heart was practically an emblem of life.) Nor did any other juice extracted from a frog’s heart have any effect. Neither did extracts from lung or liver or doses of vinegar or wine or urine, in various combinations and dilutions. Nor did lemon juice or lime juice, or extracts of oil from their peels. Nor did a jolt of electricity. Somehow, semen was special.
In keeping with that line of thought and with his picture of semen-as-chemical-potion very much in mind, Spallanzani embarked on a new round of experiments. This time his goal was to see if semen retained its fertilizing power even if it was diluted in water. To his amazement, it did, almost regardless of how far he pushed things. Diluting and diluting and diluting and then launching into seas of calculation by way of trying to find some limit to semen’s power, Spallanzani found himself staring at ever more incomprehensible results. He took a frog egg and a drop of water from a beaker of water with a tiny bit of semen stirred in. “I found the volume of the egg to the volume of the spermatic particles as 1,064,777,777 to one.”
This was astonishing, but Spallanzani’s chemical model almost fo
rced him to miss the point. If he had taken spermatozoa more seriously and recognized that it takes only a single sperm cell to fertilize an egg, his finding would have made perfect sense. He chose, instead, to try a variant of his dilution experiments. Once again, he would knock his head against a piece of low-hanging fruit. Once again, he would ignore it except to complain that someone needed to tidy up this orchard.
This second go-round relied on filters. Spallanzani dissolved frog semen in water and passed the mixture through a piece of paper. The filtered material still fertilized eggs, although not quite so well as an unfiltered mixture. The same held for a semen and water mixture poured through two sheets of paper, or three. Finally, by the time Spallanzani had poured the mixture through half a dozen sheets, it no longer fertilized eggs. But when Spallanzani took a dab of the goopy residue left behind on the stack of filter papers, mixed it in water, and painted that semen and water mix on the eggs, he ended up with as many fertilized eggs as he’d ever had.
The conclusion, in Spallanzani’s mind? Some component of semen carried its fertilizing power, and it was that component that ended up caught on the filter paper. That fit perfectly with his chemical model, and Spallanzani cheerfully recorded his findings. He never put the viscous blob on his filter paper under a microscope. If he’d looked, he would have seen it crowded with wriggling sperm cells. Perhaps he’d have wondered why.
DISMISSING SPERM CELLS AS PARASITES WAS AN ENORMOUS MISTAKE, but it was a blunder that stemmed from experimental mishap and not from ideological blindness. Along with his dilution experiments, Spallanzani had examined sperm cells with minute attention and concluded that they truly were animals. Semen samples from humans, horses, dogs, goats, bulls, sheep, fish, frogs, and salamanders all teemed with animalcules that looked and behaved just as you would expect from tiny, living organisms—they swam along purposefully, powered by their wriggling tails; they maneuvered past obstacles; they slowed down (and finally died) as they were cooled and moved about vigorously (and finally died) as they were heated, just like microorganisms that lived in pond water.
These were bona fide animals, then, and why would anyone expect animals of one species to have anything to do with sex and reproduction in the unrelated species where they happened to make their home?
Here we come to one of the most tantalizing near misses of the whole sex and babies saga. Although Spallanzani did not believe that sperm cells had any role in fertilization, he did wonder how these parasites propagated themselves. He offered a guess. Perhaps the tiny animals were passed along in semen during sex. If semen met an egg and fertilized it, the animalcules would enter the egg and make their way to the embryonic tissues that were destined to develop into genitals. (Presumably they would die off if they did not land in a congenial, male home.) There they would bide their time, waiting out the years until their new host produced semen of his own.
As dedicated and skilled an observer as he was, Spallanzani might have undertaken to test this theory. We can imagine him gazing through his microscope at a glass dish containing egg and semen. For once, he would have focused his attention not on the semen itself but on the sperm cells within it. If all had gone well, he would have become an eyewitness for the ages, the first person in history to witness sperm meet egg. What would he have made of that gigantic triumph?
Strangely, he would probably still not have concluded that sperm cells had anything to do with fertilization. More likely, he would have celebrated his correct guess that spermatozoa were parasites that made their homes in embryos. “It is rather alarming to think,” writes the medical historian Elizabeth Gasking, “that had Spallanzani really seen the penetration of the eggs by the spermatozoa, he would have regarded it as a confirmation of this hypothesis.” Seeing is believing, the saying goes, but the reverse can also hold. Sometimes believing is seeing.
CAUTIOUS AS ALWAYS, SPALLANZANI HAD ALSO LOOKED AT THE parasite question from another angle. These observations, too, conspired to mislead him. In experiments with toads, he wrote, he had almost always found their semen “very full of spermatic worms.” Twice, though, he had been surprised to find no sperm cells at all. He had dabbed some of that sperm-free semen on eggs and found that they developed into tadpoles and then into toads. In follow-up experiments with frogs, he again found semen samples that contained no “worms” but that nonetheless proved capable of fertilizing eggs. Here was more proof that worms were beside the point. Spallanzani underlined his conclusion. “My long experience in the world of microscopical animals, whether belonging to man or animals, will, I hope, vouch for me that I was not deceived in this delicate investigation.”
And yet he was deceived. The only plausible explanation is that Spallanzani, scrupulously careful though he was, had overlooked a sperm cell or two. Then, perhaps, he responded the way most of us do when experience seems to confirm our beliefs. He nodded contentedly and moved on to other questions.
The damage was substantial. So influential was Spallanzani, one historian of medicine remarks, that another full century would pass before scientists accepted the idea that spermatozoa played a role in fertilization.
ALMOST AS REMARKABLE AS SPALLANZANI’S NEGLECT OF SPERM cells was this: despite all he had discovered about how semen worked, he gave no thought to the notion that perhaps semen and egg deserved equal billing in the sex and conception drama. Instead, like his fellow ovists, he focused nearly all his attention on his beloved eggs. He fervently believed that the egg was the headline act, and semen merely a bit player whose role was limited to providing the chemical nudge that set the fertilization process in motion. (It did so by jump-starting the embryo’s heart.)
This was another profound mistake, but it, too, arose from painstaking experiments and careful deduction. After he had finished his work on the properties of semen, Spallanzani had turned his attention to eggs. How did fertilized eggs differ from unfertilized ones? After long hours with scalpel and microscope and countless rounds of poking, probing, comparing, and dissecting, the truth was plain. Unfertilized and fertilized eggs did not “differ in the least.” From that starting point, the argument rolled out with the inevitability of a proposition in logic.
Spallanzani spelled it out: If unfertilized eggs were identical to fertilized eggs, and if fertilized eggs developed into tadpoles and then into frogs, then it followed beyond the possibility of dispute that the tadpoles were already present in the unfertilized eggs. The tadpoles had been there all along! The semen’s contribution was essentially to raise the curtain and shout, Voilà!
This was, in the 1700s and well into the 1800s, an entirely mainstream view. When Leeuwenhoek died, back in 1723, the spermists had lost their last great advocate. Ever since, the egg had reigned supreme. (This was not seen as something to do with frogs only, but as a general fact of nature.) From a historical vantage point, this is extremely odd. The belief that had prevailed for millennia—that men planted seeds and women nurtured them—had nicely bolstered the self-regard of male scientists. But that theory had given way to a picture of males as all but irrelevant, even though the very scientists proposing the new view still worked in an almost exclusively males-only club and still lived in an almost entirely male-run world.
Somehow no one seemed to notice. This curious state of affairs prevailed for almost a century and a half, from Leeuwenhoek to Darwin. Throughout the plant and animal kingdom, the historian John Farley observes, “Reproduction was a uniquely female occupation in which the role of the male was very limited or even entirely unnecessary.” Babies were women’s work. So was making them.
That view was in large part due to Spallanzani. He was a brilliant thinker and an unsurpassed experimenter. He had sped ahead of all his rivals. His proof of semen’s key part in fertilization and his demolition of “seminal aura” were giant advances. But he had missed a colossal chance. He might have looked harder at the swimming animalcules under his microscope and wondered if they could be up to something. He might have thought again about the do
g he had artificially inseminated and wondered why it was, if fertilization was a story where the female played the only lead, that the puppies looked like both parents and not just their mother. He might have examined a drop of semen from his filter papers under his microscope.
He might have. But he didn’t.
TWENTY-ONE
THE CRAZE OF THE CENTURY
LAZZARO SPALLANZANI AND HIS PEERS TOOK AS THEIR MISSION sorting out the properties of life’s building blocks. But life was more than an arrangement of constituent parts. Something more was needed. Some fluid, some force, some spark had to jolt those parts to life.
Cue electricity! In the 1700s, scientists believed they had finally found, in electricity, the “vital force” that explained the gulf between life and nonlife. Which was why, in a Paris courtyard in 1772, a physicist named Joseph-Aignan Sigaud had arranged sixty nervous volunteers into a human chain, each man holding his neighbors’ hands and waiting anxiously for Sigaud to finish charging his electrical generator.
Demonstrations like this one were wildly popular. At a signal, Sigaud shocked the first man in line, who jumped in pain. As did the next and the next and the next, and so on, while the crowd whooped in delight. (The charge was strong enough to startle but not to harm.) But wait! Something had gone wrong. The shock had passed through the first six men but then stopped.
No one could think why. Sigaud tried again. Once more, the shocks and the yelps stopped with man number six. Everyone had a theory about what could be wrong with him. In the commotion, Sigaud didn’t think to try the experiment without number six’s participation. Soon the rumors converged on a single theme—the problem with the unfortunate Monsieur Six was that he lacked the animal vitality, the sexual vigor, that ordinary men possessed. In Sigaud’s words, “the young man in question was not endowed with everything that constitutes the distinctive character of a man.” In time, the great mystery reached the ears of the Duke of Chartres, who proposed a test. It so happened that among the duke’s musicians were three castrati. Would these men be susceptible to electric shocks?
The Seeds of Life Page 21
