The Great Debate: Preformation versus Epigenesis

Overview

During the 1700s in Europe, embryology was the focus of a controversial and lively debate that involved many of the greatest and most celebrated scientists and philosophers of that time. Most scientists were convinced that all embryos existed since Creation as preformed miniatures, held within their parents, ready to simply grow larger and emerge when their time arrived. But a few scientists believed that each embryo was formed gradually, structure by structure, in a developmental series that started with the undifferentiated materials of the egg. Both sides of "The Great Debate" used the subjective philosophies of the Enlightenment period, such as rationalism and vitalism, to fill the gaps created by the limitations of their eyes and early microscopes, which left them unable to advance embryology any further. The Great Debate has come to symbolize the effects that cultural and nonscientific factors can have on scientists and their interpretation of scientific facts and theories.

Background

The biology of reproduction, or the ability to recreate new individual organisms, is possibly the single most remarkable phenomenon of life, and this process has remained a central issue of biology for several centuries. Man's quest to understand exactly how a new organism forms, referred to as "generation" in the past and currently termed "embryology," challenged the ability of many of history's greatest scientists and philosophers. The difficulty of studying such minute structures left anatomists and physiologists of the seventeenth and eighteenth centuries without the requisite facts needed to understand the events and processes of embryo formation. This period of scientific research and discovery was dominated by philosophers and scientists who were deeply affected by their own subjective interpretation of the world around them. The Great Debate that followed occurred among a group of men—women scientists were very rare during this period—who were passionate about science and the natural world, and many were devoted to God and what they saw as the illumination of his divine plan. Though concerned with the specific facts of embryo development, at its core the Great Debate questioned the role of God as divine Creator and ultimate source of all living organisms.

The Great Debate: Preformation versus Epigenesis

Overview

Background

Most scientists and philosophers of the eighteenth century were preformationists, based on the works of researchers such as Joseph of Aromatari, who claimed to see miniature chick embryos in chicken eggs even before incubation (1626). Later, Jan Swammerdam (1637-1680) stated a distinct theory of preformation based on similar research with insect, chicken, and frog eggs (1669). Swammerdam's ideas on preformation were further expanded in 1674 by Nicolas de Malebranche, who postulated emboïtement, or the encasement theory, in which completely preformed embryos were arranged in a box within a box system. Preformationists believed that all living organisms, plant and animal, were created in complete but miniature form, within the eggs that each parent contained. Emboïtement postulated that all future generations existed in a long series of miniature, complete embryos held in place and awaiting their appointed time to grow and emerge. During the eighteenth century Albrecht von Haller (1708-1777), an accomplished and distinguished scientist as well as a deeply religious man, became the preeminent advocate and supporter of preformation.

On the opposite side of this debate were the epigenesists, united in their complete rejection of preformation. They sought to explain the development of the embryo as a gradual formation of embryonic structures from the undifferentiated materials of the egg. One classic version of embryo formation was postulated by Aristotle (384-322 b.c.), who observed that the chicken embryo and its internal structures were not preformed but rather formed gradually. Two thousand years later in 1651, William Harvey (1578-1657), studying chicken and deer embryos, reached the same conclusion. Harvey stated that all life comes from an egg and that the embryo builds its organs and other parts individually, in succession and due order, in a process he termed "epigenesis." And like Aristotle, Harvey also postulated that a life force, which he called the "generative principle," initiated the embryo's growth after fertilization. Later, Caspar Friedrich Wolff (1738-1794) would take the lead in researching and advocating a theory of epigenesis that would counter the preformationists and their often implausible explanations.

Impact

Interestingly, the theory of preformation that developed in the eighteenth century was based on the apparently erroneous results reported by two researchers both considered by their contemporaries to be prime founders of preformation. First, Swammerdam reported dissecting complete butterflies from chrysalids as well as from within caterpillars. He went further in not only claiming that a complete butterfly would also be found in the egg, but that all animals are preformed as minute embryos awaiting an embryological development of simple enlargement. Perhaps more scientific than Swammerdam, but similarly in error, was Marcello Malpighi (1628-1694), who made detailed observations of chicken embryos at various developmental stages, including the presence of a beating heart at 38 hours of incubation. However, Malpighi also reported finding advanced organ development in fertilized but unincubated eggs. He believed (apparently incorrectly) that these eggs had not yet begun to undergo any development but still possessed the preformed rudimentary structures of the embryo. Other scientists and philosophers used these reports to bolster their support for and establishment of preformation.

After Harvey, further support for epigenesis came from Abraham Tremblay (1710-1784), who reported on the unusual ability of the hydra to reproduce through budding and by artificial means. Tremblay cut hydras into small pieces, which re-grew into whole new individuals. This discovery sent shock waves through the preformationists and their theories, including Haller, then a preformationist influenced by his university mentor. Haller now rethought his position and predicted the eventual abandonment of preformation theory due to several overwhelming observational facts. First, Tremblay's work revealed that the hydra possessed the ability to organize itself as necessary. It was believed that this power could also be seen in the embryo, when its original fluid state gradually changed into organs and internal structures, which continue to change over time. Finally, how could it be that offspring typically resemble both of their parents if they are the result of preformed embryos from one parent or the other? Still, for most scientists, the philosophy and religious implications of preformation held sway over both the ideas of epigenesis and the observed facts of embryo formation.

Previously, when Anton van Leeuwenhoek (1632-1723) confirmed the presence of "spermatic animalcules" in semen (1677), some scientists began to argue that the sperm held the preformed embryo, and some animalculists (spermatists) even imagined that a homunculus, or very minute human, was found in the head of the sperm. Animalculists, including Haller, saw the female as merely providing a supportive and nutritive environment for the enlarging sperm-fetus. Then in 1745 Charles Bonnet (1720-1793) reported that female aphids could produce miniature aphid offspring without fertilization, giving support for ovism, or the belief that the female egg housed the preformed embryo. In 1758 Haller, influenced by Bonnet and studying chicken embryos, returned to preformationism, supporting the ovist view, thus rejecting his view of epigenesis. For a period of time, a major scientific conflict raged between animalculists and ovists, both groups doing vigorous battle despite their inability to accurately see most of the details of either germ cells or of embryo development. Instead, philosophical beliefs and rationalist ideas were substituted, even displacing the actual observed facts.

Epigenesis slowly began to gather renewed support in response to both the successes of the new mechanical laws of physics and astronomy and the apparent deficiencies and inconsistencies of the preformationist theories. With the advent of gravitational and mechanical laws, new approaches were being applied to living organisms, with the idea that matter and its "forces" could reasonably be applied to living organisms. Although epigenesists used more observational and empirical data to support their belief that embryos formed gradually from undifferentiated materials in the egg, they had great difficulty in providing explanations as to just how and why these changes actually happened. This was the great deficiency that discredited epigenesis in the minds of most scientists of the eighteenth century.

Haller was a well respected physician, anatomist, and physiologist as well as a botanist, poet, and noted writer when he concluded that the egg contained the preformed embryo. He would quickly become the foremost advocate and ardent supporter of preformation. Haller believed that science should aim to confirm the presence of God and his divine Creation, and must be carried on within the limits of religion, rejecting any theories that promoted materialism or atheism. Haller could not accept any theory in which matter itself possessed creative powers or formative abilities that were the domain of God alone. Thus Haller rejected epigenesis and embraced performation, despite the observed factual conflicts he once advocated. He argued that preformed embryos were simply too small to be seen before development, or that they were invisible until they gained their colored forms, and furthermore, there was a limit to what man could learn and understand about the world and nature, and that limit was set by God.

Pierre Louis Moreau de Maupertuis (1698-1759), already widely accomplished and celebrated for his research confirming Newtonian principles, published Venus physique (1745), in which he boldly supported epigenesis. He argued largely on the basis of the biparental human inheritance patterns he had studied in detail. Maupertuis believed that embryos resulted from the uniform mixing of particles present in the semen of the male and female, which represented all the body parts and structures. Embryos are thus composites of both parents, and the gradual formation of the embryo was the result of a cohesive natural force that directs this process. This position was a great challenge to Haller and the preformationists, who once again chose to ignore any conflicting observational data, while attacking and rejecting any idea that a force of nature not directly of the divine Creation could form or fashion a living organism, much less a human being.

Into this scientific and philosophical fray boldly stepped the youthful Caspar F. Wolff, who published his doctoral dissertation in 1758, advocating a "rational embryology" in which epigenesis could be the only process of true embryo formation. Wolff believed that only sound scientific methodology could lead to true knowledge of any biological question, and the empirical findings of detailed observation combined with the use of rational logic was the best method. Scientists, Wolff proposed, should aspire to gain philosophical knowledge based on their acquired historical knowledge. Wolff studied actual embryo formation in great detail, recognizing that embryos and their structures were not preformed but rather developed gradually from the undifferentiated egg. Wolff proposed that embryos form as a result of a solidification process of the liquid material in the egg, and this process is controlled by the vis essentialis, a life force conceived by Wolff and not very different from previous philosophers. Wolff continued to research embryology and epigenesis throughout his considerable career. But the influence of preformation would not be replaced by epigenesis until later in the nineteenth century, when it received support from another venerated German scientist, Johann Blumenbach (1752-1840), who confirmed the regeneration results of Tremblay and announced his acceptance of epigenesis.

Meanwhile, Haller responded to the challenge created by Wolff quickly and with rigor, if not the same arguments he employed previously. The preformed embryo is both too small and its components invisible to the human eye until it undergoes further growth, he countered once again. Haller and Wolff corresponded with each other by letter many times over the next decade, though neither side apparently moved the other to any degree. These two men could not have been more polar in their views: Haller, deeply convinced of divine Creation and the support that Newtonian principles gave to the existence of God's plan; and Wolff, entrenched in the German school of rationalism and in support of a modified type of vitalism. Haller, with the weight of his considerable stature, maintained general support for preformation beyond the end of the eighteenth century.

The Great Debate still serves to illustrate how arbitrary hypotheses lead to false systems of knowledge, and that scientific advancement and understanding requires sound scientific methodology, including proper experimental designs and observations, followed by empirical analysis. Both sides of this debate, striving to advance embryology, were unable to overcome the limitations of observation imposed by the size of the embryo, a situation that would not be resolved until the nineteenth century, when microscope efficiency improved dramatically. It is interesting to note further that preformation and epigenesis have been synthesized into a contemporary theory in which preformed molecular instructions, the DNA of the sperm and egg, are combined and used to create an embryo from the undifferentiated material in the egg.

KENNETH E. BARBER