Recent advances in the understanding of the major events that shape the immune recog- nition system have been remarkable. The analysis of immunoglobulin (Ig) gene organization and Ig repertoire diversification in lower vertebrates has provided new insight into this process in mammals. Similarly, the understanding of the early development of lymphocytes and of the acquisition of immunological tolerance has been aided by elegant studies in quail/chicken chimeras, using the power of the distinctive markers of the constitutive cells of these birds. Great strides have been made in understanding...