Published in the Wall Street Journal, Jun. 2, 2017
The Birth of Wisdom
It wasn’t until recently—the late 1800s—that we knew for sure where babies come from. Laura J. Snyder reviews ‘The Seeds of Life’ by Edward Dolnick.
On an autumn night in 1677, a Dutch civil servant named Antoni van Leeuwenhoek rose from his bed immediately after intercourse with his wife. He rushed to his study, lit a candle and examined a drop of his semen with his microscope. In shock he watched as tiny eels darted this way and that.
Leeuwenhoek was the first to realize that these “little animals”—sperm—existed in the semen of healthy men and were a crucial part of reproduction. But it would be almost 200 years before anyone could answer that most fundamental question: How are babies made? Edward Dolnick, former chief science writer of the Boston Globe, recounts the history of the search for an answer in his entertaining book “The Seeds of Life.”
The ancients had wondered about reproduction, of course. Aristotle opened up fertilized chicken eggs at intervals to observe the development of the embryo and mused that human embryos underwent the same kind of transformation. He believed that a human baby was initially formed from the man’s semen being “curdled” by the woman’s menstrual blood, “the same way,” he asserted, “as rennet acts upon milk” to form cheese.
In the early 1500s men began to seek a more accurate understanding of the human body by looking inside it. Mr. Dolnick lingers over the gory details of these autopsies, the slip and the stink of blood. But there are moments of loveliness too, as when he draws a connection between Leonardo da Vinci’s dissections of the human body and his portraits. “At the same time he was painting the Mona Lisa,” we learn, “Leonardo was cutting open the faces of corpses and dissecting the muscles of the mouth and lips, to sort out the secrets of the smile.”
This was the opening salvo of the Scientific Revolution. Before long other scientists and artists were also rejecting ancient teachings and starting to study and draw what they saw with their own eyes.
By the middle of the 1600s the English physician William Harvey, who discovered that the heart is a pump circulating blood around the body, was arguing that all mammals, including humans, came from eggs inside the mother. “Ex ovo omnia,” became his motto: “Everything comes from the egg.” But what he had seen during his dissections were not eggs, only small clusters of cells making up early-stage embryos in the uterus.
In 1672, Regnier de Graaf, a Dutch physician, sliced open rabbits at increasing intervals after copulation. He used a microscope to watch as the eggs burst out of the ovary; soon afterward, tiny embryos appeared in the uterus. Yet de Graaf, like Harvey, erred in thinking that he had seen eggs. What he had observed were the ruptured follicles out of which eggs emerge, now called “Graafian follicles.” Another Dutchman, Jan Swammerdam, claimed the credit for first seeing human eggs during the dissection of a woman.
Once Leeuwenhoek saw sperm, he was convinced that eggs played no role in conception. Instead, he believed the sperm burrowed into the uterus and grew into babies—just as apple seeds placed in the ground sprouted into trees. This claim sparked a long-running controversy between those who believed embryos arose from eggs (the “ovists”) and those who thought they came from sperm (the “spermists”).
In one notable experiment of the 1770s, an Italian priest named Lazzaro Spallanzani made tiny pants for male frogs that would prevent their semen from reaching the female’s eggs, which she deposits outside her body during mating. Eggs from females who had mated with the pants-clad males never developed into frogs, proving that eggs alone were not enough for reproduction: Semen was required as well. Two later researchers further showed that the crucial element in semen was the sperm.
The advent of cell theory finally brought the egg-sperm dispute to a close. By the mid-1800s scientists had realized that all plants and animals are formed of cells and that all cells come from other cells. If sperm and egg were cells, then they could be seen as equally important building blocks of new life. It was not egg or sperm, but both.
Observational confirmation of this joint contribution came in 1875, nearly 200 years after Leeuwenhoek. Zoologist Oscar Hertwig was studying sea-urchin eggs fished up from the Bay of Naples. He placed one of the transparent eggs under his microscope, where its nucleus was clearly visible. Poking a drop of sea-urchin semen near the egg, he watched as a tiny sperm cell pushed itself inside and wriggled toward the nucleus. “Suddenly,” Mr. Dolnick writes, “the two nuclei were in contact, and then—before Hertwig’s eyes—the two nuclei fused into one.”
Mr. Dolnick’s wide-ranging book sometimes reads like a salmagundi of facts and anecdotes about anything related to where babies come from—ancient fertility concoctions and abortifacients, myths, folklore and science. But it’s also an engaging and exuberant tour through centuries of thought about reproduction.
It turned out that knowing about the fusing of egg and sperm was only the first piece in the puzzle. The final mystery could not be solved until genes, chromosomes and DNA were discovered. “It wasn’t just the machinery in the cell that was passed along when a cell divided,” Mr. Dolnick reflects, “but instructions for constructing a whole array of new machines.” Biologists are still trying to fully decipher the instruction manual.