Within a few days of fertilization, the sea urchin embryo develops into a small differentiated organism consisting of about 1800 cells and capable of feeding, swimming, and the further ontogenic transformations required in the succeeding weeks of larval growth. A number of distinct cell lineages that are clearly specialized at the morphological and functional levels can be discerned in the advanced embryo, and many of these can be traced back to particular sets of early blastomeres. Classical cell lineage and experimental studies (Hörstadius 1939; for review, see Angerer and Davidson 1984) have shown that certain of these lineages appear to be specified, at least in part, in consequence of the maternal components inherited in those regions of egg cytoplasm occupied by their progenitor cells. Specification of others among the early cell lineages clearly depends on inductive interactions that occur between blastomeres during cleavage. For the molecular biologist, as for his predecessors, this rapidly developing and simply constructed embryo offers the advantages of experimental accessibility. Thus, in respect to direct molecular-level analyses of gene activity in the embryo, for both specific genes and overall transcript populations and their protein products, the sea urchin is at present the best known embryonic system (e.g., reviews of Hentschel and Birnstiel 1981; Davidson et al. 1982; Angerer and Davidson 1984)
