Fig. 1. A C. elegans adult hermaphrodite gonad arm, within which germ cells develop in an assembly-line fashion (Hubbard and Greenstein, 2000; Schedl, 1997). The somatic distal tip cell maintains a population of self-renewing germline stem cells. The first germ cells to enter meiosis form approximately 160 sperm during the L4 stage, then, during adulthood, exclusively oocytes are produced. As germ cells move away from the distal tip, they enter meiotic prophase I in the transition zone, then progress to the pachytene stage. The germline initially consists of a syncytium, in which germ cells are only partially enclosed in a membrane and share a common cytoplasmic core. The gonad bend or `loop' region, where germ cell deaths normally occur, is indicated by a line outside of the gonad. After exit from pachytene, developing oocytes begin to form discrete cells that enlarge as they move through the loop. During diakinesis, a signal from sperm directs oocytes to undergo maturation. Oocytes are then fertilized as they move through the spermatheca into the uterus.