The fate map of the early neural plate and neural fold has been established at the cephalic level by using the quail–chick marker system (Le Douarin, 1969, 1973). The experimental design comprised the replacement of definite territories belonging to the neural plate and neural folds in the chick embryo by their counterparts from quail embryos at the same developmental stage. This technique is referred to as the isotopic and isochronic exchange of preneural tissues between these two species. The various types of experiments that were carried out are schematized in Fig. 2. The possibility of distinguishing quail from chick cells by the structure of their nuclei allowed the fate of the grafted territories to be recognized at later developmental stages ranging from 3 to 9 days of incubation (E3–E9). Fig. 1 illustrates the morphological changes in the anterior neural plate and neural ridges in the chick embryo at the early somitic stages.