The library at the College of Physicians is a treasure trove of sixteenth and seventeenth-century volumes pertaining to human and animal reproduction, or “generation,” and provided invaluable resources for my dissertation research. My thesis explores early modern explanations of generation as well as medical problems associated with birth defects and gynecological disorders. During the mid-seventeenth century Aristotle’s and Galen’s hegemony over learned anatomy was challenged by a spectrum of views on how living bodies function. I examine the origins of these debates and the challenges anatomists faced in replacing traditional accounts of fetal development. My focus is on William Harvey’s Exercitationes de generatione animalium (1651), Nathaniel Highmore’s Corporis humani disquisition anatomica and History of Generation (both 1651), and treatises on generation that either informed their work or else reference them. While the College of physicians holds the largest collection of volumes that are pertinent to my research, I also discovered many useful books and dissertation pamphlets at the Chemical Heritage Foundation and the University of Pennsylvania. During the 1640s, Harvey led several anatomists in some of the earliest systematic observations of developing fetuses. The young physician Highmore was part of this group, yet in 1651 he and Harvey published very different theories of generation. Considering that they collaborated and emphasized the empirical foundations of t their accounts, the discrepancies between their explanations are striking. While Harvey clung to traditional Aristotelian vitalism, Highmore sought to provide chemical and mechanistic explanations of how bodies function. My dissertation delineates key differences between their studies of generation, traces sources of their ideas, and reveals ways in which their work provides a window onto central philosophical, medical, anatomical, and religious debates of the seventeenth century. Both Harvey and Highmore engaged in extensive and detailed analyses of earlier anatomical works. The Disquisitio and Exercitationes teem with references to Galen and Aristotle, to sixteenth-century anatomical works, and to early seventeenth-century textbooks. Highmore responds at length to Adriaan Spiegelius’s De formatu foetu (1626) and De humani corporis fabrica (1627), and Harvey cites the former treatise. In an effort to acquire a clearer understanding of Spiegelius’s views and check Harvey’s and Highmore’s claims about them, I studied the treatises, which are held at the University of Pennsylvania, Princeton, and the College of Physicians. Spiegelius discusses one of the central questions of early seventeenth-century generation: whether the mother provides the fetus with vital spirits or whether the fetus produces them itself. According to the traditional Galenic and Aristotelian conceptions of the body, vital spirits are distributed by the arterial blood. Whether or not the fetus derives arterial blood and spirits from its mother had important implications for its ontological status. Harvey followed Spiegelius in holding that the fetus produces the vital spirits itself. He also cites and agrees with Julius Caesar Arantius, who had argued in De formato foetu opusculum (1564) that the umbilical vessels are not joined to the uterine vessels, but rather terminate in the placenta. Harvey’s teacher at Padua, Fabricius ab Aquapendente, published a volume on the formation of the fetus in 1600 in which he argued adamantly against Arantius’s view. The College holds copies of Arantius and Fabricius’s treatises, which I consulted in order to understand the argument over vital spirits that plays a significant role in Harvey’s work. I am currently writing a chapter of my dissertation on this debate. Harvey wrote the Exercitationes with Fabricius’s works literally at his side. In the preface Harvey designated Aristotle as his general and Fabricius as his guide, and indeed, nearly every chapter is constructed around quotations and discussions of their work. The College of Physicians holds many editions of Fabricius’s De formato foetu (1600) and De formatione ovi et pulli (1621), which respectively treat live-bearing and avian generation. These volumes are exquisitely illustrated and I compared the images in the various editions. In addition to the engravings that were printed with the books, Fabricius commissioned paintings of human and animal fetuses and reproductive parts. He bequeathed these colored plates to the Venetian state with the stipulation that they be made available to the public. The College of Physicians holds the only known copy of the paintings related to generation. In contrast to his teacher, Harvey rejected the use of representations and prefaces his work with an argument against the possibility of creating anatomical images. I have just completed a chapter of my dissertation in which I contrast Fabricius’s colored plates and engravings, examine Harvey’s critique of images, and consider his alternative approaches to representing his observations. Moreover, I compare Fabricius’ engravings to illustrations in volumes I consulted at the College, most notably Casserius’ plates which were published in Spiegelius’s volumes, Caspar Bauhin’s Theatrum anatomicum (1605), and Highmore’s Disquisitio. In addition to studying the volumes that Harvey and Highmore reference (either to bolster their arguments or to critique), I found a considerable number of books and dissertations from the second half of the seventeenth century that discuss their works. Particularly valuable are Jean de la Courvée’s and Anton Deusing’s tracts on fetal nutrition both published in 1655, Johannes Sperling’s tract on the formation of humans that appeared the same year, H. Fabri’s 1666 volume on generation, and a collection of observations on Harvey’s Exercitationes that was edited by J. Schrader and published in 1674. In addition to the volumes discussed thus far, which focus on unproblematic conception and fetal development, I found several tracts treating sterility in men and women, malformed offspring, and complications in pregnancy. I was particularly surprised by the number of treatises on ectopic pregnancies that I came across, including those of Eisenmenger (1622), Lautier (1660), a collection of various German authors (1663), Deusing (1664), and Tiling (1670). These efforts to understand what came in the way of normal conception and birth are interesting in two different respects: they present different physical explanations on failure to conceive and carry a healthy fetus to term and in the process of doing so reveal the necessary conditions for producing offspring that resembles its genitors. Harvey was neither interested in chemical processes nor did he consider the underlying structure of parts of bodies. Highmore, in contrast, explained generation, growth and nutrition by means of small parts of matter undergoing alchemical and mechanical procedures. Unlike the Disquisitio, in which Highmore references many anatomists, in the History of Generation he mentions only two authors: Kenelm Digby and Andreas Libavius, both of whom were engaged in alchemical activities. Libavius’s claim that it is possible to reconstitute an animal or plant from its ashes plays an important role in Highmore’s views. In order to better understand Highmore’s position, I examined the first and second editions of Libavius’s Alchymia (1597 and 1606) as well as his 1615 commentaries on the work, which are all held at the Chemical Heritage Foundation. The opportunity provided by PACHS to consult the volumes that Harvey and Highmore reference and to discover works which discuss their views has greatly bolstered my dissertation research. In addition to being able to visit the member libraries, I benefitted from the colloquia on a vast spectrum of topics. I am grateful to the librarians, staff, other graduate students, and scholars who assisted and inspired me during my month among the PACHS community.