The C. elegans reproductive system. (Top) Nomarski DIC image of an adult hermaphrodite. The hermaphrodite reproductive system consists of two U-shaped gonad arms, in which germ cells develop in an assembly-line fashion from mitotic divisions at the distal end to ovulation and fertilization at the proximal end . Dotted lines surround the posterior gonad arm. (Bottom) Schematic drawing of one gonad arm. Germ nuclei are generated by mitotic divisions in response to a signal from the Distal Tip Cell (DTC) at the distal end of each gonad arm. As the nuclei move away from the DTCs they initiate meiosis and arrest at the pachytene stage of meiosis I. Around the time the nuclei reach the bend in the gonadal arm, oogenesis is initiated. The germ nuclei become fully enclosed by a plasma membrane, and the resulting oocyte grows dramatically in size. The germ cells exit pachytene, and progress through diplotene arresting for a second time in diakinesis, the final stage of meiotic prophase. The oocytes proceed in single file through the gonad arm, with the most mature oocyte present directly adjacent to the spermatheca.