nate, gap, and pair-rule proteins are then no longer needed. Their products disappear, and the genes are no longer expressed.
Similar conclusions apply to the development of the termini and the dorsoventral dimension, which also rely on initially asymmetric signals. The developmental mechanisms of the termini and dorsoventral dimension are of additional interest, because the signals bind to transmembrane receptors and activate signal transduction pathways, eventually leading to the activation of transcription factors and new gene expression. These inductions are the first to occur in the developing Drosophila egg. Approximately 100 genes and encoded gene products have been identified as necessary to establish the organization of the early gastrula. Hundreds more participate in the accomplishment of these events, but they are less well described at present. In most cases, these genes probably encode proteins required in numerous developmental processes and, hence, were not recovered under the conditions of the mutant inspections used here.