Six Scientific Developments of the 17th Century that Greatly Improved Our Knowledge of Developmental Biology

 

The question of how human life began was probably one of the most important biological questions of the Early Modern Period (1500-1800) but also, one of the most difficult to contend with because of the inability to gain access to human embryos. There was a lot of theorizing about this question, but the scientific advances that furthered understand all seemed to have taken place in a short period of time in the mid 17th century.

After which, important advances were few in number until the 19th century.

William Harvey

1. William Harvey Theorizes All Life Comes from an Egg (1651)

British scientist William Harvey is more famous for having discovered the circulation of blood in 1628. But he also contributed to science in other ways.  He was an avid experimenter, and  dissected countless animals. In 1651, he published Exercitationes de Generatione Animalium, in which he concluded that all life comes from an egg. The  biological reasoning behind his conclusion was wrong. He believed that eggs mysteriously appeared in the womb through some immaterial force. Notwithstanding his biological error, his conclusion represented an important conceptual advance. [1]

Robert Hooke's Micrographia

2. Robert Hooke Popularizes the Microscope (1665)

Lense grinders had been in the business of creating lenses since at least the Middle Ages. Researchers had used such lenses to view small objects. However, the use of lenses and microscopes had been largely ad hoc in the early 17th century. In 1665, Robert Hooke published his Micrographia, which consisted of a book devoted entirely to drawings of objects examined under the microscope, such as cork, a flea and mold. It was considered an artistic masterpiece. The Pre-Moderns were fascinated with the idea that they could finally see things that had always been denied them. Hooke’s contribution also consisted in explaining how he prepared his specimens to make them viewable. And in this way, he launched a vogue for microscopic observation. Other scientists acquired a microscope in order to be able to make their own observations, making it possible to observe small organisms. Such as embryos.[2]

Jan Swammerdam

 3. Swammderdam Discovers that the Same Organism Persists Through Various Stages (1669)

Today, we take it for granted that larvae and adult insects are the same species. This was not obvious to pre-Modern people. It was commonly believed that insects emerged spontaneously from rotting meat or other scenes of decay. A number of scientists contributed to disprove this idea; among the most important was Dutch entomologist Jan Swammerdam. In 1667, during an audience at the Académie Thévenot in Paris he took a silkworm--which he knew was about to pupate-- then he asked his audience to find the butterfly wings. And of course, they couldn’t. He then took a scalpel, cut off the silkworm’s cuticle, and revealed the silkworm’s wings. This astounded his audience. He showed that adult insects had evolved from beings that did not look anything like them and that they were anatomically, the same being, only that they grew different body parts at different stages of life.

Silkworms were not the only object of his research: he had performed dissections of countless species of insects – and animals that today we no longer count as insects, such as scorpions. His findings were published in 1669 in Historia Insectorum Generalis. [3] The image of an insect inside a chrysalis would eventually inspire theories of preformation and emboîtement.[4]

St. Niels Stensen

4. Niels Stensen Discovers the Function of the Ovaries (1667)        

The existence of female ovaries had been known since ancient times. They were termed the female testes. In 1667, Niels Stensen published Elementorum Myologiae Specimenin which he discussed the dissection of a dogfish and correctly identified the function of ovaries as the repository of human eggs. In 1675, he applied his conclusion to several other mammalian species in  ‘OvaViviparorum Spectantes Observationes.’  By the early 18th century, this understanding of the ovary was widely accepted.[5] Pope St. John Paul II would canonize Niels Stensen in 1983 and he is now recognized as a patron of science.

5. Regnier de Graaf is Credited with Discovery of the Human Egg (1672)

In 1672, Regnier de Graafian published A New Treatise Concerning the Generative Organs of Women. In it, he correctly identified the follicles that bear his name (Graafian follicle) and  posited that they contained the egg that becomes fertilized in an act of conception. This development was a crucial advancement in the understanding the history of embryology.

It was only in 1825 that the mammalian ovum was isolated by Karl Ernst von Baer.

 

6. Anton van Leeuwenhoek Discovers Spermatozoa  (1677)

Leeuwenhoek was not a trained scientist. He was a draper by trade. But  when it came to scientific matters, he was a devoted amateur. In 1677, he examined human sperm and sent a report to the Royal Society of London – translated into Latin— for fear that the discovery might disgust his readers. They were not disgusted; his findings were published in 1678. He would go on to examine the semen of various of mammals, furnishing an important piece of the puzzle concerning the mystery or reproduction. [6] This discovery generated a sense of wonder because of how strange it seemed (and frankly a lot of Early Modern Science is like that.).  

…

It would be wrong to say that there were no developments in the science of reproduction in the 18^th century. But they paled in comparison to these advances. The scientific world was caught up in the discussion over theories of preformation, i.e., the human embryo exists already in the sperm or ova. Though these erroneous theories generated some interesting discoveries, such as the parthenogenesis of aphids, they did not really contribute to the understanding of human reproduction.  

 

[1] For further reading: Matthew Cobb, Generation: The Seventeenth-Century Scientists Who Unraveled the Secrets of Sex, Life, and Growth (Bloomsbury Publishing USA, 2008), 39-48.

[2] For further reading: Boris Jardine, “Microscopes,” in A Companion to the History of Science (Wiley Online, 2016), 515–29.

[3] For further reading: Matthew Cobb, Generation: The Seventeenth-Century Scientists Who Unraveled the Secrets of Sex, Life, and Growth (Bloomsbury Publishing USA, 2008), 160-164.

[4] Sometimes preformationism is attributed to Jan Swammerdam, but this is incorrect. See Cobb, Matthew, “Swammerdam and Preformationism,” JanSwammerdam.Org, accessed May 30, 2021, http://www.janswammerdam.org/preform.html.

[5] For further reading: Enrico Marani and Wijnand Koch, The Pelvis: Structure, Gender and Society (Berlin Heidelberg: Springer, 2014), 504.  https://books-scholarsportal-info.proxy.bib.uottawa.ca/en/read?id=/ebooks/ebooks3/springer/2014-04-01/1/9783642400063#page=513.

[6] See: M. Cobb, “An Amazing 10 Years: The Discovery of Egg and Sperm in the 17th Century,” Reproduction in Domestic Animals 47, no. s4 (2012): 2–6, https://doi.org/10.1111/j.1439-0531.2012.02105.x.

The Origins of the Idea that Life Begins at Conception

 

From Sacred Embryology, by Francesco Cangiamila,
Book 1, Chapter 9, published 1745. 

The question of how human life begins was one of the most important scientific mysteries in the Early Modern era. Although we connect prenatal development to questions of abortion and genetics, in the Early Modern period, questions of development were more likely to be linked to questions of inheritance and infanticide. But regardless of the practical application of this knowledge,  the Early Moderns simply wanted to know how human beings began. And this question was one of the most resistant to investigation.

As seen in an earlier post, Aristotle had posited that human development began with a succession of souls: a vegetative soul, a sensitive soul and then a rational soul. Skepticism towards this idea began to development in the 16^th century. Natural philosophers reasoned that it was absurd to think that a body needed three souls. And they figured that in order for a human body to actualize its potential, it needed a human soul, not a vegetative or animal one. But it was only in the 17^th century that this skepticism gained enough attention to become mainstream.

Two natural philosophers are responsible for initiating a strong interest in this idea.

The first was Thomas Fienus, who taught medicine at the University of Louvain. In 1620 he wrote a book entitled De formatrice foetus in which he argues that the rational soul infuses no later than the third day after insemination.

Why only after three days? Why not immediately at conception?

Fienus, being a Catholic, had to steer clear of a heresy called traducianism. Traducianism holds that the human soul is transported through the male seed. [1]

Daniel Sennert

The second natural philosopher who popularized the idea of “life begins at conception” was Daniel Sennert, a German Lutheran, who sympathized with traducianism. In 1630 he wrote a book called Hypomnemata. He said that Fienus had been too preoccupied with satisfying Catholic doctrine and that it did not contradict the Catholic faith to consider that the soul entered the body immediately at conception.

His words were quoted approvingly by Paolo Zacchias, who was the papal physician, and considered to be the founder of Forensic Medicine. He wrote a nine volume series called Quaestiones Medico-Legales in which he commented on legal matters that required medical expertise. Before Zacchias published his opinion, he submitted it to ecclesiastical censure. Once the book obtained its approval, it was widely disseminated in 1650, as he was frequently consulted throughout continental Europe.

The only problem was that while it was considered philosophical plausible and even likely, it was not empirically confirmed. However,  every new discovery on matters of reproduction tended to reinforce this conclusion. Nevertheless, final confirmation, only took happened throughout the course of the 19th century, when scientists were able to confirm that contact between sperm and egg in various animals was necessary for fertilization to happen.

And it was during the 19th century that many countries undertook to overhaul their legislation on abortion, and they took into account this new scientific consensus.

 

[1] See Wolfgang Müller, The Criminalization of Abortion in the West: Its Origins in Medieval Law (Cornell University Press, 2012), 120-121.

Aristotle's Influence in Medieval Embryology

 

During the Middle Ages, right up until the 17th century, Aristotle's ideas about embryology predominated in the academy.

Aristotle incubated chick eggs and dissected the embryos at various stages of gestation. As a result of his studies, he concluded that the embryo was formed from the mixture of male seed and female menstrual blood. The male seed imprinted the active principle, the female role was more passive.

Aristotle was also influential because of his ideas about the human held. He believed there were three kinds of souls: vegetative, for plants; the sensitive soul for animals; and rational souls for humans.

Aristotle believed that the human embryo underwent a successive animation; first it received a vegetative soul, at which time, it could perform the most basic of organic functions, such as drawing nutrition and growth. Then this soul died and was replaced with a sensitive soul, once the embryo experienced circulation. Then, once the soul had all the body parts of an adult, the sensitive soul died and received the rational soul of a human. Male embryos were believed to be ensouled at 40 days, because it was at about this time that male genitalia could appear; females were thought to be ensouled at 80 days, because at that time, it could be confirmed that the female was not a male as no male genitalia had appeared. In French Early Modern Law, the embryo was considered a human being at 40 days; abortion at that time was considered a homicide. But just because abortion was not considered a homicide, doesn't mean it was not considered a sin. It was still considered a form of interference with life.

Aristotle was considered the foremost scientist of all time until well into the Renaissance, and his conclusions were considered unassailable, until the advent of the Scientific Revolution well into the 17^th century. Dissections and other empirical investigations seemed to prove him wrong. And many questioned the idea that an organism could have three souls. 

The Unborn Child in History

 

This is the first anatomically correct image of an unborn child ever produced in the West. It was drawn by Leonardo da Vinci in 1515.

This drawing would only be published in 1770 when it was discovered in Windsor Castle. [1]

Only in the 18^th century did anatomically accurate images of the unborn begin to be produced and distributed: by William Hunter and his anatomical atlas; by the artist d’Agosty and by Thomas Soemerring and his wax models of the unborn in all stages of life.

Before the development of these pictures, the unborn were depicted as small adults, like they were in Jane Sharp's 1671 manual on midwifery:

The unborn were not accurately represented because they were not the objects of dissection.

Dissection began to be practiced in medical schools in Italy in the late Middle Ages. Contrary to what some believe, there was no Church teaching against dissection; however there was a strong cultural opposition to it. Medieval Europeans did not like the idea of their loved ones being cut up before being buried, even if it was for an autopsy.    This is why dissections used the bodies of executed criminals or paupers. Most of these were men. If women were condemned to death, they would “plead the belly” in order to escape being executed until after childbirth. Very few pregnant women faced capital punishment. Even when pregnant women were executed because pregnancy was not confirmed, it did not automatically mean that a physician was waiting on them to explore their body.

You might wonder: with all the miscarriages that women underwent, surely there would have been an ample supply of fetuses to examine.

But here’s the thing: until the development of modern obstetrics, pregnancy and childbirth was almost exclusively under the supervision of women. Men almost never got involved. There may have been times a surgeon was called in to terminate obstructed childbirth. Or they might have been needed for some other complication. But they virtually never got involved in miscarriage or most births. Nobody thought to preserve the products of miscarriage to send to a physician. It was considered to be a purely private, female affair.

Leonardo da Vinci was only able to draw this particular child because it appears that he practiced dissection.

When one reads about the unborn child in history, one must always bear in mind that the unborn child is a mystery and has been, and to a large degree still is.

[1]Lawrence D. Longo, Wombs with a View:Illustrations of the Gravid Uterusfrom the Renaissancethrough the Nineteenth Century (Springer, 2016), 20. https://books-scholarsportal-info.proxy.bib.uottawa.ca/en/read?id=/ebooks/ebooks3/springer/2017-08-17/2/9783319235677.