Fixed lineages are the extreme manifestation of the interrelations between cell division pattern, spatial organization and cell differentiation occurring in embryonic development. These relationships are discussed in the light of recent new detailed descriptions of lineages, experimental perturbations and genetic analysis. Lineages could be phylogenetically old devices under the control of specific genes that are required to organize the morphological space. Such genes would be different from those involved in the specification of lineage alternatives. Specification genes operate independently from each other and are expressed in a combinatorial way in different cells and lineages. Since their activity is reversible during development the possibility exists for changing these combinatorial associations during development and evolution. The classical notion of a hierarchical cascade segregation of histotypes along lineages is discussed.