Determination and differentiation of tissues during embryogenesis of multicellular organisms relies on differential utilization of genetic information in different cell types at different stages of development, resulting in unique patterns of gene expression that impart the necessary morphological, biochemical, and functional characteristics to different cell lineages. One important approach to the analysis of embryonic development has therefore focused on the identification of tissue and stage specific gene products, both at the level of gene expression and of cellular protein products. Monoclonal antibodies have been successfully used in the identification, analysis, purification, and cDNA cloning of developmentally regulated antigens in many organisms, including insect, amphibian, avian, and mammalian species (1 –10 ).
