The gonads develop from cells of two different origins; the cells of the germline (primordial germ cells) derive from the so-called pole cells, whereas the somatic cells originate from the mesodermal layer of abdominal segments a5, a6 and a7. The primordial germ cells or pole cells, are set aside very early in development. They bud off at the posterior pole of the syncytial embryo. During embryonic development, they are internalized with the midgut, migrate through the midgut epithelium and along its basal surface, into the lateral trunk mesoderm, where they will contact somatic gonad precursors of mesodermal origin. During their extensive migration, a substantial number of pole cells get lost. After completion of germ band shortening, the embryonic gonads are visible as two dorsolateral cell clusters at the level of a5.