The polarity of the dorsoventral axis in the Drosophila embryo is defined by an extracellular signal.

Twelve maternal effect loci are required for the production of Drosophila embryos with a correct dorsoventral axis. Analysis of mosaic females indicates that the expression of the genes nudel, pipe, and windbeutel is required in the somatic tissue, presumably in the follicle cells that surround the oocyte. Thus, information coming from outside the egg cell influences dorsoventral pattern formation during embryogenesis. In transplantation experiments, the perivitelline fluid from the compartment surrounding the embryo can restore dorsoventral pattern to embryos from females mutant for nudel, pipe, or windbeutel. The positioning of the transplanted pervitelline fluid also determines the polarity of the restored dorsoventral axis. We propose that the polarizing activity, normally present at the ventral side of the egg, is a ligand for the Toll receptor. Presumably, local activation of the Toll protein by the ligand initiates the formation of the nuclear concentration gradient of the dorsal protein, thereby determining dorsoventral pattern.