Fig. 1. Anatomy of MSP signaling. (A) Diagram of the hermaphrodite reproductive tract. Oocytes undergo meiotic maturation in an assembly line fashion in response to MSP signaling. At ovulation, the distal constriction of the spermatheca dilates, the oocyte enters and is fertilized. (B) Electron micrograph of the spermatheca. Spermatozoa are unable to enter the proximal gonad because the constriction of the distal spermatheca provides a barrier. Some spermatozoa enter the uterus with embryos, and must then crawl back into the spermatheca so they can fertilize oocytes. (C) Spermiogenesis is the process during which non-motile spermatids become fertilization-competent motile spermatozoa with a pseudopod. Spermiogenesis occurs when spermatids enter the spermatheca during the first ovulation in hermaphrodites, or as they enter the uterus during mating. In A, a few spermatids are shown remaining in the gonad arm, as is typically seen on the first two days of adulthood. These spermatids will enter the spermatheca in the next few ovulations.