umans have always known that sex was necessary to procreation. But what happened between intercourse and birth was a profound, indeed total, mystery. Most surely guessed that semen had something to do with it, but no one knew what, and this gave rise to the male-centered notion that men contributed a “seed” that was sown in the uterus. Of course, that couldn’t help explain how it was that many children resemble their mothers more than their fathers.
As Edward Dolnick shows us in his fascinating new book, The Seeds of Life, it would take 400 years—from the time of Leonardo da Vinci, who made wonderful anatomical drawings, to the late 19th century—to unravel the mystery. Dolnick is the former chief science writer for the Boston Globe and the author of The Clockwork Universe, a splendidly readable history of the scientific revolution of the 17th century.
In the Middle Ages, the ancient authorities held sway—and in the case of birth that meant Aristotle, who lived in the fourth century b.c.e., and Galen, a Greek physician who lived in Rome in the second century c.e. Aristotle thought conception occurred when a man’s semen mixed with a woman’s menstrual blood. Galen speculated that women emit a fluid during intercourse that, together with male semen, began the process. Indeed, Galen thought that men and women were anatomically identical, just with women having inverted genitalia.
The great biologist William Harvey (1578–1657), who had first explained how blood circulated throughout the body, was not convinced. He carefully dissected chicken embryos at various stages of development. Late in his life he published a book called Disputations Touching the Generation of Animals. Unlike many of his contemporaries, Harvey believed female mammals produced eggs, like female birds.
But, as with the capillaries he described, Harvey had never actually seen one. As Dolnick points out, while the human egg is the largest cell in the body, it is still only about the size of the period at the end of this sentence. (Sperm are the smallest of human cells, only about one-millionth the size of a human egg.) Harvey believed firmly that “ex ovo Omnia” (from the egg everything), and was thus the first of the “ovists.” Those who believed that semen was the crucial thing became known as “spermists.”
The Dutch scientist Regnier de Graaf (1641–1673) continued the exploration of how human life comes to be. First he demonstrated the anatomy of testicles. Dissecting rabbits, he noted that structures in the ovaries, now known as Graafian follicles, always burst open when rabbits became impregnated. He also noted that the number of embryos that appeared a few days later usually equaled (and never exceeded) the number of ruptured follicles. Hence, De Graaf concluded, the ovaries were the source of the eggs Harvey had posited. Shortly before his untimely death at 32, De Graaf notified the Royal Society in London that a neighbor of his in Delft had developed a microscope that was far more powerful than earlier models, which had been hardly better than magnifying lenses.
With his new microscope, the businessman and scientist Anthony van Leeuwenhoek was able to magnify objects up to 300 times. He soon made one of the most astonishing scientific discoveries of all time, the microscopic living world of what he called “animalcules,” from the Latin for tiny animals. Everything, from pond water to food to his own saliva, he found teeming with life.
Boundlessly curious, one evening, after having had sex with his wife, he leapt out of bed and put his own semen under the microscope. To his utter astonishment, it, too, was alive with wriggling creatures.
We know them to be spermatozoa, the male sex cells. But having seen how ubiquitous were the animalcules he had discovered in such infinite variety, Leeuwenhoek simply assumed that spermatozoa, too, were parasites and he ignored them. As Dolnick notes, the history of solving the mystery of sex (he might have added, the whole history of science) is full of incidents “where a brilliant investigator notices a smoking gun, picks it up, mulls it over, and then tosses it aside as irrelevant.” The mistake went unquestioned for well over a hundred years. Even as late as the 19th century, encyclopedia drawings of various human parasites included finely rendered drawings of sperm cells next to tape worms and liver flukes.
The “ovists” doubted that semen was even necessary to procreation, or played only a minor role. In one of the most elegant experiments of all time, an Italian scientist named Lazzaro Spallanzani (1729–1799) showed it was vital. He sewed together small silk pants, wax coated, and put them on male frogs, most species of which have external fertilization. Frogs without the pants had no trouble fertilizing eggs as they emerged from the female. Those wearing the amphibian boxer shorts could not. He then took some of the semen from the pants-wearing frogs, fluid that had been trapped by the pants, and brought it in contact with the eggs. They developed normally into tadpoles. (This was the world’s first instance of in vitro fertilization.)
He mixed semen and water and then passed it through filter paper. It still fertilized eggs. Only when he’d passed it through six filters did it no longer fertilize the eggs. Then he mixed some of what the filter paper had kept back with water and brought it in contact with the eggs. Fertilization happened. Spallanzani was on the cusp of a great scientific discovery. But, like Leeuwenhoek before him, he dropped the ball. Had he put material held back by the filter paper under a microscope, he would have seen that it was made up of highly concentrated spermatozoa. But he didn’t. He shared the view that spermatozoa were parasites and thus played no role in fertilization.
Only in the 19th century was the conundrum finally solved. In 1824, the young French scientist Jean-Louis Provost and the Swiss Jean-Baptiste Dumas observed that while every fertile animal, whether mammal, bird, or fish, has spermatozoa in their semen, infertile ones, such as mules, have none. Further, if the spermatozoa are killed, then no fertilization takes place. No spermatozoa, no fertilization; therefore spermatozoa had to be the causative agent.
In 1827, a German scientist named Karl Ernst von Baer finally found a mammalian egg under his microscope, a fact that caused him to “shrink back as though struck by lightning.” (He also gave spermatozoa their name, which means “animals of the semen.”) In the 1830s, the fact that all living things are constructed of cells began to take hold. And so, rather than being animals in their own right, sperm were simply sex cells, as were eggs.
Finally, in 1875, a German scientist named Oscar Hertwig had a sea urchin egg under his microscope to which he added a drop of semen. A sperm cell emerged and its head penetrated the egg’s membrane, somehow it made its way to the nucleus and the two nuclei fused into one.
The Seeds of Life is science-history writing at its very best. It tells a great story that reads like a mystery novel. But Dolnick also uses the story to demonstrate how science advances: through curiosity, brilliant insight, analogy, logic, experimentation, and hard work. Equally, he shows how it can be retarded by unexamined assumptions, ad hominem arguments, misplaced ego, and stubborn adherence to outworn theories.