Development

Aristotle's conception of development was shaped by what he could observe—fluids and semen at conception and the gradual emergence of form in the course of development. It is therefore only logical that the next major changes in the philosophical and scientific analysis of development are connected with emerging possibilities of observation during the seventeenth century. One instrument, in particular, played a central role in discussions about development—the microscope. The microscope allowed for the first time analysis of the constitution of those observable fluids at the beginning of development. Looking at semen with his single-lens microscope, Anton van Leeuwenhoek could see structures in the head of the spermatozoa. But what did those structures represent? In the wake of the scientific revolution, a mechanical approach dominated the sciences and medicine. William Harvey had found a mechanical solution to the circulation of blood, and generations of anatomists had analyzed the form and function of the human body in similar terms. In this context of mechanical ideas, Leeuwenhoek's observations took on a specific meaning. For some of his contemporaries, the structures inside the sperm thus represented a smaller, already preformed version of the adult organism, called an "homonculus" by some. Development then was simply a mechanical unfolding and subsequent growth of structures already present at the very beginning in either the sperm or the egg. Others, such as Harvey, continued to advocate the epigenetic position of Aristotle. These epigenesists also claimed that their views only described what they could observe.

Clearly, observations were ambiguous and often fit theory-driven expectations. Preformists were committed to a mechanical and materialistic explanation. They did not want to rely on any form of entelechy or vital force in order to account for development and were also opposed to ideas and reports of spontaneous generation. Epigenesists, on the other hand, were committed to the action of a vital force in nature. They also emphasized the role of observation and pointed out that several facts, such as the existence of hybrids or "monsters," could not easily be explained within the preformist framework. The influence of metaphysical commitments in shaping the interpretation of observations can best be seen in the mid-eighteenth-century debate between Caspar Friedrich Wolf and Charles Bonnet. Both looked at chick embryos at the same stage (twenty-eight hours after fertilization). Both described in detail what they saw—no clearly defined beating heart, for instance. And both arrived at radically different conclusions. For Wolf it was obvious that the heart would only form later due to the agency of a vital force (vis essentialis); Bonnet, on the other hand, concluded that even though he could not see it, the heart must nevertheless already be there.