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Eggs of many organisms contain a store of mRNA which supports protein synthesis during early embryonic development and various regions of the egg cytoplasm are not identical with respect to developmental potential. I investigated the extent to which sea urchin embryogenesis results from a progression of developmental events directed by the embryo, or an expression of a pre-formed maternal program. By the use of two-dimensional electrophoresis I demonstrated that cellular determination during embryonic development at the 16-cell stage is not accompanied by qualitative changes in the distribution within the embryo of abundantly-synthesized proteins, virtually all of which are coded by sequences present in the egg. Using two-dimensional gel electrophoresis, nucleic acid hybridization and molecular cloning, I demonstrated that there is restricted expression of paternal gene mRNA sequences in interspecies hybrid embryos. In some cases, this is due to a posttranscriptional perturbation of gene expression in the hybrid embryos.