At the time little was understood about the reproduction of silkworms—or of any other animals, for that matter. Margaret Cavendish could publish a verse on the life cycle of the silkworm suggesting that several butterflies were generated from the decay of a single dead caterpillar, not realizing that the caterpillar was turning into a moth. Even more specialized students of insect anatomy and behavior held or had held a similar view about the reproduction of the silkworm, including the naturalist Ulisse Aldrovandi (1522–1605), one of Malpighi’s predecessors at the University of Bologna. However, during his study of the silkworm Malpighi saw that the silkworm becomes the moth—after the worm becomes a chrysalis, what emerges is the same creature, now transformed into a new form. Malpighi became interested in the origin of the parts of the developing insect, noticing that “the parts belonging to, and destined to be used by the moth,” are visible in earlier stages of the creature. This led him to study the development of vertebrates—starting with the embryo of a chick.
As Malpighi knew, centuries earlier Aristotle had conducted a study of chick development by opening up chicken eggs each at a different day after fertilization to examine the changes that occurred in the embryo day by day. Aristotle had correctly noted three stages of chick development: after three days of incubation, the chick heart can be observed; after ten days all parts of the body can be detected; after twenty days the chick is covered in downy feathers and is ready to emerge from the egg; it already makes the “cheep-cheep” sound of the young chick.
Using his anatomical methods, as well as a microscope, and with his mechanistic expectations in place, Malpighi was able to give a description dramatically more accurate than that of Aristotle or later investigators. Malpighi observed and described numerous details of the parts of the developing chick: the neural fold (the precursor to the nervous system), optical vesicles (which develop into the eyes), the cardiac tube (the precursor to the heart), and aortic arches (structures that turn into the major arteries). He studied the chick heart, noting that it was observable to the naked eye as a red, pulsating speck before any of the other parts of the chick became visible. However, when viewed with a microscope, the heart was visible even before the liquid propelled through it turned red; it could be seen solely through its motion, which the microscope revealed. This examination led him to the cautious claim that the entire chick “lies concealed in the egg” right at fertilization, though he admitted that he had not come to a definite conclusion about this.
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During this period natural philosophers were puzzling over how new organic beings were made, especially in the case of viviparous animals, those giving birth to live offspring. Today that knowledge seems obvious: sperm meets egg, forms an embryo, and a baby is the eventual result. But in the seventeenth century little was known about how offspring were created, and how they developed, a mysterious process given the overarching term “generation.” No one had seen either sperm or egg, or any part of the process by which they meet. It was commonly believed—even by natural philosophers—that insects arose spontaneously out of dirt, excrement, or carrion, that women could give birth to animals or “monsters,” and that certain kinds of birds grew on trees. In the 1660s the fellows of the Royal Society discussed how to produce baby vipers from the powdered parts of vipers, and even managed to procure “a glass-jar, full of the powder of the bodies of vipers, and a gallipot full of the powder of only the hearts and livers of vipers.” Checking on these vessels over time, they concluded that though the viper powder was now foul and full of “little moving creatures” (maggots, perhaps), it had not generated any baby vipers.
Since mating in domesticated animals can be controlled by separating the sexes or by castrating the males, and since human virgins do not become pregnant, it was clear, of course, that reproduction was linked to the sex act. But what exactly happened during and after the sex act was unknown, as was whether copulation was necessary for generation in insects, birds, reptiles, and other creatures.
European views on generation were still dominated by ancient thought on the topic. In his work On the Generation of Animals, Aristotle had concluded that while the mother’s body contains the material necessary for creating her offspring, she requires the father’s semen to start and guide the process. The male’s semen provides the shape, or form of the embryo, as well as the cause of its formation, while the female’s provides merely the stuff that is formed into an embryo by the semen. (The generation of insects was different—they arose spontaneously from decay, Aristotle believed.)
Later, the physician Hippocrates (ca. 460–370 BCE) held that both the female and the male contribute a kind of semen—the ejaculate of the man and the menstrual blood of the woman—and that both are responsible for the forming of the embryo. In the second century CE, Galen proposed a view similar to that of Hippocrates, but according to Galen the female’s contribution is not her menstrual blood but an internal secretion similar to the male’s semen. This claim had the benefit of suggesting to some writers on the topic that the woman must reach sexual climax, just as the man must, in order to conceive—and so for centuries the idea that sex should be a mutually enjoyable activity had a scientific imprimatur.
By the seventeenth century, not much had changed since ancient times. But by turning his microscope to the study of chick embryos, and seeing what was invisible to the naked eye—the embryo heart before blood began flowing through it—Malpighi had shown that the microscope could be a valuable tool in investigating generation. He was not the only one who realized this. In the Dutch Republic, the former university classmates Swammerdam and De Graaf were each pursuing microscopical studies of the reproductive organs, trying to find the hidden mechanisms of generation.
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After he had settled in Delft around 1667, De Graaf continued his anatomical work on the sex organs that he had begun in Leiden, spending much time in the anatomical theater. In 1668 he published a work showing that the testicle is not composed, as was then generally believed, of a spongy, pulpy, or “glandulous” substance, but that it instead is a tangle of minute “vessels.” To prove his point to the Royal Society, De Graaf sent them a specimen: “the testicle of a dormouse, unravelled by my method.” An image of that specimen was published in the Philosophical Transactions in October of 1669.
The publication of De Graaf’s work set off a vicious dispute with his one-time fellow student Swammerdam and their teacher Johannes van Horne. Van Horne was also doing research on the testicle, and had enlisted Swammerdam’s aid in trying to clarify its structure by the use of Swammerdam’s unique method of injecting vessels with colored wax to make them stand out as well as preserve them for future study. Van Horne had realized, like De Graaf, that the testicle was composed of tiny tubes or vessels. Having learned of De Graaf’s publication shortly before it appeared, Van Horne rushed into print a summary of his own research showing the “thready” nature of the organ. He explained that his failure to publish before De Graaf was due to the “laziness of the engraver” and hinted that his former student had stolen his work. De Graaf took offense and responded in kind, accusing his former teacher of stealing his work. Swammerdam threw oil on this priority fire by recalling that he had had discussions with De Graaf about Van Horne’s researches. Things grew nastier when De Graaf published a treatise on the female reproductive organs in 1672. De Graaf had conducted a series of dissections on female rabbits shortly after mating. Unlike earlier writers since Hellenistic times, De Graaf claimed that the sex organs of females of the higher animals were not “inverted testicles,” but were “ovaries,” structures containing eggs, or ova in Latin. (Harvey had previously observed eggs, and had argued that embryos arise from the egg alone, coining the slogan ex ovo omnia—from the egg comes everything. But he had ignored the ovaries, thinking they were rudimentary organs, like male nipples, serving no purpose.) De Graaf again gave no credit to Van Horne, who had earlier described the female ovary and had even suggested that female huma
ns have eggs. Swammerdam attacked De Graaf once more for neglecting to acknowledge those who had priority for the discoveries. This time, Swammerdam claimed that he was the true discoverer of human eggs.
After more back-and-forth sniping between the two men, during which time each tried to convince the Royal Society fellows of his priority, the Royal Society decided to end the dispute over the discovery of eggs once and for all. A committee was appointed and after some delay reported that the first person to have seen eggs in viviparous organisms was not Van Horne, or De Graaf, or Swammerdam—but Steno, who had seen eggs (but without being certain what they were) in his 1667 paper describing his dissection of a dogfish. This report had the effect of emphasizing that priority in discovery went to those who published first. It also underlined the importance of a view of generation that was universal, one that would explain not only human generation but also the generation of other viviparous animals.
By the time the committee published its conclusions, De Graaf had already died, perhaps of suicide, at the age of only thirty-two. He is buried next to his infant son in the Oude Kerk in Delft. One of his last scientific acts was sending the letter introducing Leeuwenhoek to the Royal Society. Leeuwenhoek would later report that he had heard at the time of De Graaf’s death that Swammerdam and De Graaf had had “such a sharp verbal altercation that not only did the latter fall ill, but this was also followed by his death”—that the dispute with his former friend had killed De Graaf. Years later Leeuwenhoek himself would make the most astounding discovery related to generation, one that would utterly transform our understanding of the origin of new life.
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By the time Oldenburg received Leeuwenhoek’s first letter, the Delft civil servant was forty-one years old. He had progressed from using a magnifying glass to look at fabric, to making his own single-lens microscopes, to turning his instruments on small organic and inorganic objects around him: tiny cheese mites, minuscule grains of chalk. In that first letter, Leeuwenhoek described mold, the sting of the bee, and the “nose” of the louse. The Royal Society secretary encouraged Leeuwenhoek to continue his studies and to send his results to London. He specifically suggested that Leeuwenhoek use his microscope to study blood and other bodily fluids. De Graaf had proposed in his letter of introduction that the Royal Society put some difficult questions to Leeuwenhoek and give him suggestions for further investigations, and over the years the fellows of the Royal Society did just that. Many of Leeuwenhoek’s discoveries were due to investigations he began at the urging of the “curious Gentlemen dabblers” in London.
The Royal Society’s interest in his investigations was just the encouragement Leeuwenhoek needed, and he threw himself into this study, often using himself as his own experimental subject. In his second letter to the Royal Society (the first addressed directly to Oldenburg), he described how a louse feeds, which he observed by putting “a hungry Lowse upon my hand, to observe her drawing blood from thence.”
The Lowse having fixt her sting in the skin, and now drawing blood, the blood passeth to the fore-part of the head with a fine stream, and then it falls into a larger round place, which I take to be filled with Air. This large room being, as to its fore-part, filled about half full with blood, does then propel its blood backward, and the Air forward again; and this is continued with great quickness, whilst the Lowse is drawing blood.…
His concentration must have been intense to allow him to remain still while the louse sucked up his blood, and to watch the progress of his vital fluid through the innards of the insect! Later, Leeuwenhoek would submit himself to more exposure to lice, placing newly hatched lice on his hand and allowing them to feed there, carefully charting the entire process. His interest in lice piqued, Leeuwenhoek went on to an even more gruesome experiment.
After locating three adult lice, he placed them on his calf, and then put on a tight stocking so that the lice were bound to his leg. He left the stocking on and refrained from washing his leg for six days. Removing the stocking at this time, Leeuwenhoek found over eighty eggs stuck to his leg hair, but no young lice. Leaving the eggs on his flesh, he somehow managed to get himself to put the stocking back on (I can’t imagine doing so). After ten days had elapsed from the start of the experiment, he took off the stocking and, as he told a correspondent, “I saw at least twenty-five young lice running about.” Understandably, “this spectacle of all the said young lice filled me with such aversion to the stocking that I threw it, along with all the lice in it, out the window.” He then rubbed his whole leg with ice to make sure no lice could remain, rubbed with ice a second time, and finally put on clean stockings. After his careful description of this experiment, Leeuwenhoek calculated the reproduction rate of lice, concluding that on the body of a “Poor Person, who does not have a change of linen or garments,” from two pairs of lice ten thousand young can be generated in only eight weeks. He signed off this letter with a pun about his “lousy discourse.”
Using one’s own body as an experimental laboratory was common practice in those days. Not long before Leeuwenhoek first wrote to the Royal Society, Isaac Newton was in Cambridge performing a particularly macabre self-experiment (or so he claimed)*2 in order to test his theory of the perception of colors:
I tooke a bodkine [needle] & put it betwixt my eye & [the] bone as neare to [the] backside of my eye as I could: & pressing my eye [with the] end of it (soe as to make [a] curvature in my eye) there appeared severall white darke & coloured circles.
After gingerly inserting a sharp needle between his eyeball and orbital socket, Newton gently moved the needle, changing the needle’s pressure on the eyeball. Newton’s aim was to explore the anatomy of the eye and how that anatomy influences how we see. Newton was also trying to discover the way in which apparent sensations might in fact be the product of imagination and to answer the question whether what one saw might be controlled by the nerves, and thus perhaps by the soul itself, rather than by some mechanical process of experience—questions that had earlier been raised by Descartes. Newton would later present papers to the Royal Society based on these grisly self-experiments.
Some of Leeuwenhoek’s self-experiments were more enjoyable than those involving lice and needles. One evening he drank “two pounds of good French wine,” followed the next day by another pound and a half of “good French, new-Rhine, and old Upper Moselle wine.” On the next day he consumed a pound and a half of tea, quickly, to make himself sweat. He examined his sweat with the microscope to see whether the salt particles he had previously found in wine would come through the skin’s pores during perspiration. He did not find salt particles in his sweat. (Most likely, he had to repeat this experiment several times to be sure.)
Leeuwenhoek also observed his blood with his microscope, after drawing a sample from his thumb. He expected, perhaps, to find salt particles there. Instead, he observed numerous small “red globules,” floating in what appeared to be a clear and crystalline fluid (it had, on first examination, looked white and “wheylike,” but later he realized it was clear). To his surprise, Leeuwenhoek had discovered red blood corpuscles.
Others had seen, with the aid of microscopes, that there was something in the blood: Kircher had reported seeing tiny “worms” in the blood of plague victims, and Borel had described dolphin-shaped “insects.” They had probably seen stacks of adhering corpuscles known as rouleaux. Malpighi had discerned single red blood corpuscles as early as 1665, writing in 1666 that he had observed “red atoms” in the transparent blood vessels of a frog—but he had thought them to be fat globules, like those he had seen elsewhere in the body. Swammerdam saw them as well, without knowing what they were. Leeuwenhoek was the first to realize that these globules were particular to the blood, and that they gave blood its color. He initially reported his findings in a letter to Constantijn Huygens, reminding his friend that he had told him about this observation “when I was at your house some time ago.”
Over the following years Leeuwenhoek would continue h
is investigation of red blood corpuscles, attempting to understand why blood clots and why it turns bright red when exposed to the air. He would later tell Hooke that he had “many times repeated Observations of my own blood.” He looked at other specimens of blood besides his own, including the blood of a rabbit. Leeuwenhoek compared the size of a red blood cell to that of a “coarse” grain of sand, which he had determined to be about 1/30 of an inch. Since he said that one hundred red corpuscles were less than the size of a coarse grain of sand, that puts his estimate at less than 1/3000 of an inch, or less than 8.5 microns, which is impressively close to the modern measurement of 7.7 microns. Leeuwenhoek told Hooke that he knew his readers would find his calculated size “incredible,” and so they did.
Others were trying to see these tiny corpuscles for themselves but, as Leeuwenhoek soon discovered, they were having trouble replicating his observations. Even when he sent to Christiaan Huygens in Paris “some of the small Glass-pipes” he had used to make the observations, the younger man was unable to see corpuscles, instead observing only other, macroscopic particles, probably the rouleaux. It would take four long years before the corpuscles were observed by others—first at the Leiden shop of the Musschenbroek brothers, who were already making and selling Leeuwenhoek-style single-lens microscopes by the 1670s, and later by Swammerdam and Christiaan Huygens. Each of these observers—like Leeuwenhoek—would see the red blood cells as globular, influenced by Descartes’s popular corpuscular philosophy, although they are in fact flatter and more concave.
Eye of the Beholder: Johannes Vermeer, Antoni van Leeuwenhoek, and the Reinvention of Seeing Page 28