An Acknowledgement of 18th Century Preformationist Contributions to the Field of Developmental Biology
In 1590, Hans and Zacharias Janssen invented the first compound microscope. The Dutch father and son team used two lenses at the top and bottom of a cylindrical tube to create a magnification up to 9x, enough to see details on insect larva. It was not until decades later that the microscope was used for scientific work. In 1661, Marcello Malpighi observed capillaries in frog lungs. A few years later, Robert Hooke used the compound microscope to write Micrographia, detailing various insects' anatomy and famously identifying plant cells. The microscope earned acclaim after another Dutch scientist, Antonie Van Leeuwenhoek, discovered bacteria and protozoa while disproving the theory of spontaneous generation. At this point in the late 17th and early 18th century, microscopes had been significantly improved, and magnifications could reach nearly 300x, generating a field view of approximately one millimeter, small enough to see red blood cells. The advent of the compound microscope fundamentally spurred the scientific revolution of the early modern period for the fields of biology and anatomy/physiology.
With the development and improvement of microscopy technology, researchers began to uncover the mysteries of life, and two main theories regarding animal creation arose: preformationism and epigenesis. During the 18th century, the field of biology became polarized into preformationists and epigeneticists. Preformationists believed that germ cells contained all of the same structures as adult organisms. This “homunculus” or “animunculus” then grew fully formed features from the existing structures throughout gestation. epigeneticists believed germ cells developed features sequentially to form a mature organism. Caspar Friedrich Wolff later refuted the theory of preformationism when he determined that developmental growth occurs from the apex of an axial structure. Modern geneticists and developmental biologists may be quick to disregard preformationist theories in the 18th century; however, they were well supported by existing literature and technology. While the theory of preformationism has been disproven, it is important to give credit to preformationist scientists who advanced the study of developmental biology.
The theory of epigenesis had existed since before 300 BCE, when Aristotle first theorized heredity and embryonic development in his book, Generation of Animals. Long before spermatozoa or ovum could be visualized under a microscope, Aristotle predicted semen contained genetic material essential for the development of features inside of the mother. While his theory is not precisely in line with the modern understanding of embryonic development, his rejection of inheritance of acquired characteristics and acknowledgment of maternal and paternal contribution to organism creation was surprisingly accurate, especially since he hypothesized over 2,000 years ago.
The theory of epigenesis grew and evolved as scientific tools advanced. Scientists were able to visualize microscopic structures and processes and eventually identify germ cells. William Harvey changed the perspective of epigenesis in his response to Aristotle's theories, Exercitationes de Generatione Animalium, published in 1651. Harvey observed the development of a chick embryo and concluded that growth must occur via epigenesis since structures do not emerge simultaneously. Since the ovum was not identified until 1827 by Karl Ernst von Baer, Harvey's interpretation of fertilization was theoretical; however, it laid the groundwork for other epigeneticists to disprove theories of preformation., By the late 18th century, the theory of preformationism had been scientifically refuted, and eventually, preformationism was absolutely and unequivocally disproven by Hans Driecsh in the 19th century.
Decades earlier, preformationists had also looked at chick development. Marcello Malpighi, the same researcher who had discovered capillaries, microscopically observed chick embryo development and concluded structures must be performed. His determination was not based on seeing fully formed structures within the developing chick embryo but rather that the developing features resembled those of a fully formed chick. This was later elaborated on by Charles Bonnet when he studied regeneration in earthworms. He argued that features did not develop sequentially but were already formed and simply too small to see. Referring back to Malpighi's work with chick embryos, he said, "If we were able to see the germ enlarged, as it is when small, it would be impossible for us to recognize it as a chick." Bonnet’s conclusion can be reasonably interpreted as profound for the time. If one observes human embryo development, the fetus at just four weeks gestation has key distinguishing features similar to that of a fully grown adult. While the initial development of an organism does not come from preformed structures, Bonnet did recognize anatomical similarities between early-stage development embryos and adult organisms.
The theory of preformationism was further supported by Bonnet’s discovery of parthenogenesis, or “virgin birth,” in aphids. After this discovery, Bonnet was convinced that the ovum must contain the animunculus since asexual reproduction could not result in an offspring created solely by the mother otherwise. This made him vulnerable to ridicule from not only epigeneticists but other preformationists, who subscribed to spermism as described by Nicholas Hartsoeker in 1694. Hartsoeker's initially suggested the idea of preformationism after observing sperm cells under a microscope. As one of the first scientists to see sperm cells, he suggested that if one could see beneath the germ membrane, one may see a humanoid structure that is further developed during gestation. His theory sought to provide a hypothesis for the long-standing question of heredity. Followers of his ideology became known as spermists and were often mocked with caricatures of homunculi in sperm cells.
Modern scientists are often quick to dismiss early modern preformationist theories, but technology, as it existed, did not allow closer examination of development. Preformationism was a very plausible theory during the 18th century, given that there was no concept of DNA and little understanding of coded data in general. While staunch advocates for their hypothesis, preformationists recognized the limitations of their tools and acknowledged that they simply subscribed to a theory.
Despite refutation of preformationism in the late 18th and early 19th century, preformationists made significant contributions to the field of developmental biology, particularly concerning embryonic development. Recognizing and charting structure development during gestation, as Bonnet and Malpighi had done with chick embryos, is now key for early identification of congenital malformations. Preformationists contributed largely to the understanding of organism anatomy. Anatomists Charles Bonnet and Jan Swammerdam significantly advanced the field of entomology and proved that insects have organs, a revolutionary discovery at the time. It is important to recognize 18th-century preformationists as legitimate scientists who based their theories on the existing biological rationale. One incorrect theory should not discredit the life work of an entire group of pioneering and devoted scientific researchers.
The challenges faced by 18th-century researchers using early compound microscopes are best understood using pictures of microorganisms under relative magnifications. One can fully appreciate the effort that went into interpreting images of low quality and drawing conclusions regarding development and heredity.
References  Poppick, Laura. “Let Us Now Praise the Invention of the Microscope.” Smithsonian.com. Smithsonian Institution, March 30, 2017. https://www.smithsonianmag.com/science-nature/what-we-owe-to-the-invention-microscope-180962725/.  Ibid.  Wills, Matthew. “The Evolution of the Microscope.” daily.jstor.org. JSTOR, March 27, 2018. https://daily.jstor.org/the-evolution-of-the-microscope/.  The Editors of Encyclopaedia Britannica. “Antonie Van Leeuwenhoek.” Encyclopædia Britannica. Encyclopædia Britannica, inc., October 20, 2019. https://www.britannica.com/biography/Antonie-van-Leeuwenhoek.  Ibid.  Newmark, Phillip. October 9, 2019.  Ibid.  “Wolff, Caspar Friedrich.” Complete Dictionary of Scientific Biography. Encyclopedia.com, December 3, 2019. https://www.encyclopedia.com/people/science-and-technology/cell-biology-biographies/caspar-friedrich-wolff.  Newmark, Phillip. October 9, 2019.  “The Embryo Project Encyclopedia.” On the Generation of Animals, by Aristotle | The Embryo Project Encyclopedia. Accessed December 4, 2019. https://embryo.asu.edu/pages/generation-animals-aristotle.  Newmark, Phillip. October 9, 2019.  Gregory, Andrew. “Renaissance Influences.” Encyclopædia Britannica. Encyclopædia Britannica, inc., September 6, 2019. https://www.britannica.com/biography/William-Harvey/Renaissance-influences.  Pinto-Correia, Clara, and Stephen Jay. Gould. The Ovary of Eve: Egg and Sperm and Preformation. Chicago: University of Chicago Press, 1998. https://ebookcentral.proquest.com/lib/wisc/reader.action?docID=408565#. 2-3.  Andrew, “Renaissance Influences.”  Oppenheimer, Jane M. “Karl Ernst Von Baer.” Encyclopædia Britannica. Encyclopædia Britannica, inc., February 24, 2019. https://www.britannica.com/biography/Karl-Ernst-Ritter-von-Baer-Edler-von-Huthorn.  Oppenheimer, Jane M. “Hans Adolf Eduard Driesch.” Encyclopædia Britannica. Encyclopædia Britannica, inc., October 24, 2019. https://www.britannica.com/biography/Hans-Adolf-Eduard-Driesch.  Newmark, Phillip. October 9, 2019.  Newmark, Phillip. October 9, 2019  Ibid.  Ibid.  Ibid.  Ibid.  “The Embryo Project Encyclopedia.” Nicolaas Hartsoeker (1656-1725) | The Embryo Project Encyclopedia. Accessed December 4, 2019. https://embryo.asu.edu/pages/nicolaas-hartsoeker-1656-1725.  Newmark, Phillip. October 9, 2019  Ibid.