Vertebrate development is characterized by extensive and complex cell migrations, during which cell interactions occur that are responsible for changes in cell behavior, fate, and terminal differentiation. In order to unravel these events, cells must be tagged with an easily detectable, permanent marker. The combination of quail and chick cells provides such a marker (Le Douarin, 1969). The marker is based on the presence in quail cell nuclei of a nucleolus-associated mass of heterochromatin, which is absent from chick nuclei. Chimeras are constructed surgically in ovo by introducing or substituting a selected rudiment or territory from one species into the other. The chimeras, constructed during the second to fifth day of incubation depending on the pattern, are allowed to develop for various periods, and eventually to hatching. Selected topics in which such chimeras have been instrumental will be described here: development of the peripheral nervous system studied through exchanges of neural tube and neural crest segments between the two species; central nervous system mapping analyzed with similar transplants performed at the level of brain vesicles resulting in the transfer of behavioral or genetic traits; analysis of the development of the immune and hemopoietic system relying on grafts of organ rudiments, the stroma of which is colonized by extrinsic stem cells; origin and traffic of these stem cells, development of immune tolerance, as well as formation of the vascular tree.