 In the late 19th and early 20th century, the topic of development was salient in controversies surrounding vitalism, such as the disagreement between Wilhelm Roux and Hans Driesch over how to explain ontogeny (Maienschein 1991). Roux thought that a fertilized egg contains inherited elements that represent different organismal characteristics. During the process of cellular division, these elements become unequally distributed among daughter cells leading to distinct cell fates. Driesch, in contrast, held that each cell retained its full potential through division even though differentiation occurred. Although this issue is often understood in terms of the metaphysics of life (vitalism versus materialism), Driesch’s interpretation of development and the autonomy of an organism had epistemological dimensions (Maienschein 2000). The explanatory disagreement involved different experimental approaches and divergent views on the nature of differentiation in early ontogeny (e.g., to what degree cells are pre-specified). A familiar philosophical theme running through these discussions, both epistemological and metaphysical, is the status of  reductionism in biology.  Through the middle of the 20th century, embryology—the scientific discipline studying development—slowly transformed into developmental biology with a variety of reworked and recalcitrant elements (Berrill 1961). In conjunction with the issue of reductionism, a key aspect of this history is the molecularization of experimental (as opposed to comparative) embryology (Fraser and Harland 2000), with a concomitant emphasis on the explanatory power of genes (see the entry on  gene  and  Section 3.1).  This complex and fascinating history, including interrelations with medicine and reproductive technology, has been detailed elsewhere (see, e.g., Oppenheimer 1967; Horder et al. 1986; Hamburger 1988; Hopwood 2019; Maienschein 2014; Maienschein et al. 2005; Gilbert 1991; Embryo Project in Other Internet Resources).