Lymphoid mononuclear cells from chick embryos at stage 16 were collected prior to fetal liver and thymus genesis to study the differentiation and function of the hematogenic yolk sac and to detect whether CD2 occurs on the surface of lymphoid mononuclear cells.The phenotype and functional activity of the cell surface protein E receptor and the ultrastructure of embryonic E+ cells were compared with those of mature T cells.Our results indicate 99.36% homology between the E receptors of embryonic lymphocytes and mature T cells.Other similarities,including molecular distribution,motivation,the ability to form an erythrocyte rosette,the structure of the receptor-ligand complex,and the conformation of the signal channel,were detected between embryonic lymphocytes and mature CD2-expressing T cells.These results indicate that CD2 is already expressed prior to fetal fiver and thymus genesis and that its expression is not dependent on the thymic microenvironment.