 A developmental mechanism is a mechanism or process that operates during ontogeny (see McManus 2012 for discussion). At least two different types of developmental mechanisms can be distinguished (Love 2017a): molecular genetic mechanisms (signaling or gene regulatory networks;  Section 3.1)  and cellular-physical mechanisms (cell migration or epithelial invagination;  Section 3.2).  Philosophical explorations of mechanisms in science and mechanistic explanation have grown dramatically over the past two decades (Craver and Darden 2013; Glennan and Illari 2017; Illari and Williamson 2012). Among different accounts of scientific mechanisms, four shared elements are discernable: (1) what a mechanism is for, (2) its constituents, (3) its organization, and, (4) the spatiotemporal context of its operation. Developmental explanations seek to characterize these four elements through various experimental interventions. Together these elements provide a template for characterizing the two different types of developmental mechanisms.