Avian embryos provide all sorts of opportunities for the study of higher vertebrate development, especially because an experimental model, in which cells from two species are combined, has been proposed as the basis of a marking technique (1 ,2 ). Cell-marking techniques are essential in developmental biology to trace cell precursors and their progeny and thus establish their behavior and fate. These markers must be nonleaky and stable as cells divide and must not interfere with normal development. The quail-chick labeling method meets all these requirements.
