 Ariane Dröscher (2014) traces the history of conceptual change concerning stem cells via lineage tree diagrams, a form of visual representation common to studies of development and of evolution. Lineage diagrams track relations between generations of reproducing entities. From Haeckel’s speculative image of the Tree of Life and Weismann’s introduction of “cytogenetic tree diagrams”, Dröscher examines this cell-centric way of understanding development from the mid-nineteenth century onward. Scott Gilbert (1991) shows that branching tracks have been used to represent cell development since the late nineteenth century, most prominently in cell lineage diagrams that track cell pedigrees and division events. In these models, branch-points represent cell division events and branching tracks represent genealogical relations among cells. Other cell characteristics are also represented, such as position within the developing embryo, morphology, and developmental fate. Waddington’s landscape is a model of this type (see  Section 8.1).  Fagan (2013a) uses this modeling approach to characterize cell development in general. Any process of cell development produces a cell lineage tree with a specific topology, representing developmental and reproductive relations between cells. Different stem cell concepts correspond to models that differentially constrain the space of possible topologies for cell lineage trees. This framework provides a systematic representation of the many different varieties of stem cell.