 Results of stem cell experiments prompt re-examination of now-standard assumptions about the nature of development. At early stages of mammalian development when embryonic stem cells are most plentiful, in the blastocyst stage just before implantation, stem cells can be harvested and cultured to become a large number of different kinds of cells (Thomson et al. 1998). In theory, they have the capacity to become any kind of differentiated cell, but it is impossible to prove that conclusively (Fagan 2013b). Position in the organism and with respect to other cells seems to be decisive in directing differentiation. Although genetics provides information about the range of possibilities, regulation of gene expression involves diverse factors—and stem cells’ plasticity and context-dependence fits a more epigenetic view. Cell reprogramming, for example, is a method of producing stem cells (induced pluripotent stem cells) from more differentiated cells, effectively reversing normal development (Takahashi &amp; Yamanaka 2006). This contradicts any strict principle of development as irreversible and internally-directed, undermining the notion of a fixed program for development (Brandt 2010). Melinda Cooper (2003) examines how central concepts of stem cell research (potency, differentiation, form, and regeneration) fit within the long tradition of theorizing about epigenesis and self-organization, dating from seventeenth-century revivals of Aristotelian embryology.