William Harvey and John Hunter: Investigations in Generation.

Although living more than a century apart, William Harvey and John Hunter were preoccupied with investigating many of the same biological quandaries: how does blood circulate in the body? How do animal’s bodies compare to humans? And above all, what are the origins and processes of generation? It is this final question that was explored in two fundamental texts in the history of human and animal generation, William Harvey’s Exercitationes de generatione animalium (On Animal Generation, 1651) and the posthumous Essays and Observations on Natural History (1861) by John Hunter. Both anatomists utilised the works of their predecessors on the matter, however it was their empiricist approach – that is, that knowledge can only be derived from first-hand observation – that distinguished their investigations from others.

The publication of On Animal Generation had two purposes: firstly, to present Harvey’s research into the origins and development of generation, and secondly, to prove the importance of empirical study. Seventeenth-century physiologists generally held the opinion that life began either by “spontaneous generation” – the theory that lowly animals such as insects and reptiles were born from decomposing matter – or for mammals, that life began as a tiny, fully formed body in the sperm that simply grew over the gestation period (known as “preformationism”). Harvey criticised individuals who supported these classical theories, not because they were necessarily incorrect, but because they blindly believed texts without conducting their own investigations.

Example of “preformated” human in the sperm. From: Hartsoeker, Nicolas. Essay de dioptrique, Paris: Anisson; 1694, p 230.

Harvey’s claim was that all life, no matter plant, insect, or animal, generated from an egg or seed, and that blood was the origin of life in the generation of animals. To prove this, he observed the development of multiple hen’s eggs over two weeks and recorded his examinations with precise details, which he later documented in sixty-two subchapters of On Animal Generation. On the fourth day of incubation Harvey observes (p 90):

in the center almost of it (the yolk), there leapeth a capering bloody point, which is so exceeding small… like the most obscure and almost indiscernible spark of fire.

Over the next days the “channels of blood” began to pulse, form the heart and liver, and later the other organs and limbs. By cracking eggs and peeling off their shells daily, Harvey had therefore witnessed with his own eyes that the chick did not begin life with a preformed anatomy. Life as he observed it, began with blood.

“Ex ovo omnia” (“Everything from the egg”). Frontispiece of Exercitationes de generatione animalium, William Harvey (1651), from the RCS England Library.

The theories presented in On Animal Generation were debated throughout the next century by both scientists and philosophers who conducted their own experiments on generation; some agreed with Harvey, and others upheld the preformation argument. It took another one hundred years however for generational theory to be revived with as much vigour as Harvey and was done so by John Hunter, a leading advocate for comparative anatomy and empiricism,

Essays and Observations on Natural History is a posthumously published collection of Hunter’s hand-written notes that detail his anatomical experiments on a variety of species. A large portion of the manuscript collection is dedicated to observations on the generation of animals and humans, particularly the embryonic development of fowl, which fascinated Hunter throughout his career. He recorded his replication of Harvey’s egg experiment over a series of manuscripts which also included the instructions for preparing the eggs and conducting the experiment. Hunter surpassed his predecessor however by using magnifying aids during his research and recording his observations in both text and illustrations.

“Of the Different Methods Necessary to be Taken to Examine the Progress of the Chick in Incubated Eggs” by John Hunter, from the RCS England Archives.

“Of the Blood’s Motion in the Chick” by John Hunter, from the RCS England Archives.

Unlike Harvey, who believed that true empiricism only involved the naked eye, Hunter used a microscope to reveal parts of the embryo that were otherwise too small to see. He noted that the very first cluster of embryonic cells was still indistinguishable – microscopes were still relatively primitive at the time – yet in agreement with Harvey, Hunter records that the first sign of development was the membrane becoming vascular; in other words, thin lines of blood became visible in the yolk, thus proving that blood supply was indeed a vital component of generation and possibly the origin of life. By employing the microscope however, Hunter could further identify the “bloody streaks” as veins and arteries and observe how the vessels aided the development of the brain, spine, major organs, and finally the limbs.

To compare his chick observations with the generational developments of other animals and humans, Hunter employed Jan van Rymsdyk to illustrate the specimens in minute detail, beginning with the freshly fertilised yolk and ending with a fully developed chick (which has been further anatomised for the viewer). In 1756 Hunter further commissioned the artist to produce two large-scale pastel pieces that illustrated the stages of egg development, which were likely employed as teaching aids in his anatomy classes. It was these students that would ensure the Hunterian (and Harverian) method of empirical research would continue into the nineteenth century, especially by using microscopes to investigate cell development.

Notable followers include Everard Home, who presented his microscopic observations of blood circulation in tribute to Hunter, and Joseph Lister, whose passion for microscopic anatomy led to great improvements in the field of bacterial cell theory.

“Plate LXVIII, Stages of generation in chicken eggs”, from Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy, John Hunter, from the RCS England Archives.

Plate LXXVI, Stages of generation in geese eggs from Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy, John Hunter, from the RCS England Archives.

“Ova of the Hen.” Jan van Rymstyk. 1758-9. RCS England Museum Collection.

The RCS England Library holds multiple copies of William Harvey’s Exercitationes de generatione animalium (in addition to his other seminal work, De motu cordis), as well as an extensive collection of Hunter’s work which extends to the Museum and Archive collections. To consult our collection, please email us at library@rcseng.ac.uk to make an appointment, or to view the Archive collections, please email archives@rcseng.ac.uk.

Georgia Wallace, Library Assistant.