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▲ FIGURE 15-17 Dorsoventral axis determination in Drosophila. This process relies on two signal systems, one in follicle cells and the other in the oocyte or embryo or both, plus a proteolytic cascade within the perivitelline space. (a, b) Movement of the oocyte nucleus creates an initial asymmetry ( 1). The dorsal location of the oocyte nucleus ultimately results in the production of Pipe protein only in ventral follicle cells ( 2|- 5). (c) Subsequent events in the perivitelline space along the ventral surface generate a gradient of active Spätzle ( 6- 9). By this time, the egg has been fertilized and the embryo is a syncytium with many nuclei; the region around only one nucleus is shown. Activation of Toll on the embryo's surface

Embryo (ventral)

Embryo (ventral)

Gastrulation defective*

Gastrulation defective

Gastrulation defective*

Gastrulation defective

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