The ontogeny of lymphoid cells seems the most appropriate place to start. The early events in T and B cell ontogeny are still confusing. There seems to be no agreement on the data and on the semantics of the question of progenitor vs. stem cells. Nevertheless, we are beginning to understand more about progenitor cells which are committed to particular cell lines, and about stem cells in the sense of having almost unlimited capability of giving rise to undifferentiated progeny. An important future development will be to determine the nature of the substances that attract stem cells and which are produced by specialized thymus epithelium, and perhaps by the bursa. The way stem cells recognize these signals is an important question to answer. Not predictable from mammalian models has been the observation that there is a lack of cells called into the bursa even before the signal for entry of stem cells has been shut off. A likely model suggested that after a certain point in development there were no longer any cells capable of migrating into the bursa and becoming B cells. A fascinating possibility is the suggestion that a cell comes into the bursa, is not committed, then can still wander into the thymus. This cell does not appear to have B cell characteris tics; that is, immunoglobulin is not expressed on its surface.

Inhalt
Lymphoid Development.- Ontogeny.- Ontogeny of Myelopoietic Precursor Cells in the Chicken Embryo.- Ontogeny of Hemopoietic Colony-forming Units in the Chick Embryo Spleen.- Differentiation of the Primary Lymphoid Organs in Avian Embryos: Origin and Homing of the Lymphoid Stem Cells.- Cell Transplantation into Immunodeficient Chicken Embryos - Reconstituting Capacity of Different Embryonic Cells.- Migration Patterns of Avian Embryonic Bone Marrow Cells and Their Differentiation to Functional T and B Cells.- B Cell Maturation.- Unique Aspects of Immunoglobulin Expression During Early B Cell Differentiation in the Chicken.- Analysis of Immunoglobulin Receptors During Antigen-induced Maturation of B Cells.- Lymphocyte Antigens, Receptors, and Factors.- The Production of a Lymphocyte Inhibitory Factor (LyIF) by Bursal and Thymic Lymphocytes.- Detergent Solubilization of B-lymphocyte Immunoglobulin.- Association of Lymphocyte Alloantigen Genotypes with Levels of Immune Responses.- Rosette Formation in Chicks: with Special Reference to QRBC-Rosettes.- Regulation of the Immune Response.- Tolerance.- Protein-induced Neonatal Specific and Non-specific Immunosuppression.- The Role of the Adherent Cell in Antigen-Antibody Complex Induced Immune Unresponsiveness.- Suppressor Cells.- Infectious Agammaglobulinemia: Suppressor T Cells with Specificity for Individual Immunoglobulin Classes.- Further Characterization of the Sensitizing Bursa Cells and of the Target for Suppression in the Transfer of Agammaglobulinemia.- Regulatory Lymphocytes from Anti-y Bursectomized Agammaglobulinemic Chickens.- Regulatory Lymphocytes in T Cell Functions in Chickens.- Inherited Immunodeficiency.- Inherited Iiranunodeficiency in Chickens: A Model for Common Variable Hypogammaglobulinemia in Man?.- MajorHistocompatibility Comple.- Structure.- Isolation and Partial Characterization of the Major Histocompatibility Antigen in the Chicken.- Some Recent Recombinants at the B Locus.- The Structure of the Major Histocompatibility Complex of the Chicken.- Immune Response.- Immune Response Genes in Chickens: The Multifarious Responsiveness to (T,G)-AL.- Immune Response and Adult Mortality Associated with the B Locus in Chickens.- Cell Interactions.- Histocompatibility Requirements for Cellular Cooperation in the Chicken.- Allo-aggression in Chickens: Analysis of the B-Complex by Means of GVH Splenomegaly and by Inhibitory Antibodies.- Cell Transfer Studies with the DK/OR Inbred Chicken Lines.- Disease.- Role of the Major Histocompatibility Complex in Resistance to Marek's Disease: Restriction of the Growth of JMV-MD Tumor Cells in Genetically Resistant Birds.- The Influence of the Major Histocompatibility Locus on Marek's Disease in the Chicken.- Genetic and Cellular Control of Spontaneous Autoimmune Thyroiditis in OS Chickens.- Inbred Lines.- Syngeneic Inbred Lines of Chickens and Their Use in Immunogenetics.- Tumor Immunity.- Induction in B2/B2 Chickens of Immunity to Transplantable Carcinogen-induced Fibrosarcomas Mediated by T-cell Monocyte Cooperation: Role of Delayed Hypersensitivity to Unrelated Antigens.- Role of Tumor Antigen in Vaccine Protection in Marek's Disease.- Immune Responses and Prevention of Lymphoid Leukosis Tumors in Chickens Fed an Androgen Analog.- Immunoglobulins.- Allotypes.- Genetic Polymorphism of Chicken 7S Immunoglobulins.- A Note on Unexpected Chicken 7S Immunoglobulin Allotypes.- Chicken High Molecular Weight Immunoglobulin (IgM) Allotypes: Localization on the Heavy Chains and Proposed Nomenclature.- Binding Site.- Comparison of theMicroenvironment of Chicken and Rabbit Antibody Active Sites.