The reproductive organs comprise the gonads (ovaries, testes), the genital ducts with their accessory structures, and the external genitalia. The gonads contain the gametes, descendants of the embryonic pole cells, and support cells (sheath cells and pigment cells in males, follicle cells in females) that are of mesodermal origin. The genital ducts and external genitalia are derivatives of the genital imaginal disc. At the blastoderm stage [stage 5], the pole cells (pc) form a cluster of 34-37 cells at the posterior pole of the embryo (Sonnenblick 1950; Campos-Ortega and Hartenstein 1985; see Foe et al.; Spradling; both this volume), which is formed by the anlage of the posterior midgut rudiment (pmg).

During gastrulation, the pole cells become incorporated into the lumen of the posterior midgut rudiment [stage 8]. At the time when the posterior midgut rudiment loses its epithelial structure, the pole cells migrate out of the midgut rudiment. In the stage 11 and early stage 12 embryo, they are located between the dorsal surface of the posterior midgut rudiment and the overlying mesoderm. During germ-band retraction [stage 12], the pole cells form an elongated cluster on either side. This cluster is in close contact to a set of mesoderm cells that will later give rise to the gonad sheath (follicle precursor cells, fp; see Bate, this volume).

After germ-band retraction [stage 15], pole cells and the surrounding mesodermal sheath cells form the gonads (go). They are located laterally in segment A5. Only a fraction of the former pole cells become incorporated into the gonads; the remaining pole cells become lost in the yolk. The genital disc (gd) arises as a narrow ridge of cells located in the ventral midline between the denticle belt of A8 and the anal plate.