The quail-chick cell-marking technique consists of constructing chimaeras in ovo by grafting quail embryonic rudiments into chick embryos or vice versa. Quail and chick cells can be recognized at any time in the chimeras owing to the structures of their interphase nuclei that can be visualized after cytological staining of the DNA. Recently, antibodies that recognize species-specific determinants carried by either neuronal cell bodies or neurites of one or other of the species have greatly enhanced the analytic capacities of this technique, particularly for studying brain development. This article describes the application of the quail-chick chimaera technique to the study of the development of the cerebellum and the optic tectum in the avian embryo. The use of the interspecific chimaeras for behavioural studies is also illustrated by experiments in which certain genetic characteristics of the quail song pattern have been transferred to the chick by neural transplants.