CHIMAERAS (or “allophenic individuals”) let us see the pedigree of cells. For instance, a blastocyst compounded from a black-coated-mouse embryo and a white-coated-mouse embryo may develop into a striped or piebald adult mouse1. In other adult tissues, other markers (such as isozymes) reveal a similar conglomerate of two cell types2. Naturally occurring mosaics, such as those which arise according to the Lyon hypothesis through random X-inactivation in females, provide further examples3,4. In every case, the chimaeric pattern records something of the developmental history of the individual. Indeed, it has been claimed that the assortment of cell types in the chimaeric differentiated tissue provides some crucial evidence about the mechanisms of differentiation and morphogenesis. In particular, Mintz1 and Tettenborn et al.4 have sought to infer the initial numbers of specific tissue progenitors from observations of how often, in mosaic animals, those tissues are not mosaic, but homogeneous. We shall analyse this type of inference, and argue that the conclusions to which it properly leads do not have quite the significance that has been suggested.