 In addition, mapping the tree of embryogenesis involves assigning to each specific, identifiable cell type, a particular branch, or place, in the tree.This requires knowing the “ancestry” of each cell type, that is, which cell type preceded it in the development process.This information can be deduced by observing in detail the distribution and placement of cells, by type, in the developing embryo, and by also observing, in cells growing in culture (“in vitro”) any differentiation events, should they occur for whatever reason, and by other means.