cork had anything in common with human flesh. In fact, Hooke imagined that cork cells are mere passages for transporting fluids around the cork tree. The cell here is just a passive void in the plant’s fabric, a very different notion from the modern concept of an entity filled with the molecular machinery of life.
More provocative were the observations of the Dutch cloth merchant Antonie van Leeuwenhoek in 1673 that living organisms can be of microscopic size. Leeuwenhoek saw such “animalcules” teeming in rainwater – mostly single-celled organisms now called protists, which are bigger than most bacteria. You can imagine how unsettling it was to realize that the water we drink is full of these beings – although not nearly so much so as the dawning realization that bacteria and other invisibly small organisms are everywhere: on our food, in the air, on our skin, in our guts.
Leeuwenhoek contributed to that latter perception when he discovered animalcules in sperm. That was one of the substances that the secretary of London’s Royal Society, Henry Oldenburg, suggested the Dutchman study after receiving his communication on rainwater. Leeuwenhoek looked at the semen of dogs, rabbits and men – including his own – and observed tadpole-like entities that “moved forward with a snake like motion of the tail, as eels do when swimming in water.” Were these parasitic worms? Or might they be the very generative seeds themselves? They were, after all, absent in the sperm of males lacking the ability to procreate: young boys and very old men.
Here we can see the recurrent tendency to impose familiar human characteristics on what is evidently “of us” but not “like us”. The French physician Nicolas Andry de Boisregard, an expert on tiny parasites and a microscope enthusiast, claimed in 1701 that “spermatic worms” could be considered to possess the formative shape of the fetus: a head with a tail. In 1694, the Dutch microscopist Nicolaas Hartsoeker drew a now iconic image of a tiny fetal humanoid with a huge head tucked up inside the head of a spermatozoa: not, as sometimes claimed, a reproduction of what he thought he could see, but a figurative representation of what he imagined to be there.
The “homunculus” in sperm, as imagined by Nicolaas Hartsoeker.
This was one of the most explicit expressions of the so-called preformationist theory of human development, according to which the human body was fully formed from the very beginning of conception and merely expands in size: an extrapolation down to the smallest scale of the infant’s growth to adulthood. According to this picture, the female egg postulated by Harvey continued to be regarded in the prejudiced way in which Aristotle had perceived the woman’s essentially passive role in procreation: it was just a receptacle for the homunculus supplied by the man.
That was, though, a different view to the one Harvey envisaged, in which the body developed from the initially unstructured egg. Harvey, like Aristotle, thought that semen triggered this process of emergence, which Aristotle had imagined as a kind of curdling of a fluid within the female. The idea that the embryo unfolds in this way, rather than simply expands from a preformed homunculus, was known as epigenesis. These rival views of embryo formation contended until studies with the microscope, particularly investigations of the relatively accessible development of chicks inside the egg, during the eighteenth and early nineteenth century put paid to the preformation hypothesis. Embryos gradually develop their features, and the question for embryologists was (and still is) how and why this structuring of the tissues comes about.
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The observations of early microscopists did not, then, engender a belief that life is fundamentally cellular. That cells are a general component of living matter was not proposed until the early nineteenth century, when the German zoologist Theodor Schwann put the idea forward. “There is one universal principle of development for the elementary parts of organisms,” he wrote in 1839, “and this principle is in the formation of cells.”
Schwann developed these ideas while working in Berlin under the guidance of physiologist Johannes Müller. One of Schwann’s colleagues in Müller’s laboratory was Matthias Jakob Schleiden, with whom he collaborated on the development of the cell theory. Schleiden’s prime interest was plants, which were more easily seen under the microscope to have tissues made from a patchwork of cells, their walls constituting emphatic physical boundaries. This structure wasn’t always evident for animal tissues (especially hair and teeth), but Schwann and Schleiden were convinced that cell theory could offer a unified view of all living things.
Schleiden believed that cells were generated spontaneously within the organisms – an echo of the notion of “spontaneous generation” of living matter that many scientists still accepted in the early nineteenth century. But he was shown to be wrong by another of Müller’s students, Robert Remak, who showed that cells proliferate by dividing. Remak’s discovery was popularized – without attribution – by yet another Müller protégé, Rudolf Virchow, who tends now to be given the credit for it. All cells, Virchow concluded, arise from other cells: as he put it in Harveian manner, omnis cellula e cellula. New cells are created from the division of existing ones, and they grow between successive divisions so that this series of splittings doesn’t result in ever tinier compartments. Virchow proposed that all disease is manifested as an alteration of cells themselves.
Virchow was the kind of person only the nineteenth century could have produced – and perhaps indeed only then in Germany, with its notion of Bildung, a cultivation of the intellect that encouraged the emergence of polymaths like Goethe and Alexander von Humboldt. Virchow studied theology before taking up medicine in Berlin. While establishing himself as a leading pathologist and physician, he also became a political activist and writer and was involved in the uprisings of 1848. As if to demonstrate that nothing was simple in those times, this eminent biologist and religious agnostic was also profoundly opposed to Charles Darwin’s theory of evolution, of which his student Ernst Haeckel was Germany’s foremost champion.
Virchow thus had fingers in many pies; but in his view they were different slices of the same pie. The influence of politics, ideology and philosophy on science is always clearer in retrospect, and that’s nowhere more apparent than in the physiology of the nineteenth century. For Schwann, “each cell is, within certain limits, an Individual, an independent Whole”: an idea indebted to the Enlightenment celebration of the individual. The cell was a living thing – as the physiologist Ernst von Brücke put it in 1861, an “elementary organism” – which meant that higher organisms were a kind of community, a collaboration of so many autonomous, microscopic lives, in a manner that paralleled the popular notion of the nation state as the collective action of its citizens. Meanwhile, Schwann’s conviction of the cellular nature of all life, implying a shared structural basis for both plants and animals, was motivated by his sympathy for the German Romantic philosophical tradition that sought for universal explanations.
For Virchow, this belief in tissues and organisms as cellular collectives was more than metaphor. It was the expression writ small of a principle that applied to politics and society. He was convinced that a healthy society was one in which each individual life depended on the others and which had no need of centralized control. “A cell … yes, that is really a person, and in truth a busy, an active person,” he wrote in 1885. “What the individual is on a grand scale, the cell is that and perhaps even more on a small one.”
Life itself, then, showed for Virchow how otiose and mistaken was the centrist doctrine of the Prussian statesman Otto von Bismarck, who at that time was working towards the unification of the German states. Virchow attacked Bismarck’s policies at every opportunity and denounced his militaristic
