A formal description of cell lineage and a hypothesis: all cell lineages are fixed

The basic proposition of this paper is that all normal cells arise as a result of a fixed number of cell divisions, and this number is genetically determined for each cell. The purpose of the paper is to refine the proposition, offer evidence in support of it, and show how it can be exploited in understanding normal differentiation, speciation and malignancy. al and very specific description for cell lineage is proposed, which is applicable to any organism, and carries no implication about mechanisms of cell differentiation. The description is a variation on the usual conventions for describing lineage. Heuristic methods are proposed for analyzing cell lineages in situations where direct observation of developing cells is not possible, for example in large organisms.