Overview of the fertilization process in C. elegans (each panel consists of the Nomarski image and an explanatory cartoon). (a) shows an oocyte ready for ovulation into the spermatheca, where the sperm are stored. The mutants discussed in the paper produce a few fertilization-competent sperm (shown in the cartoons in grey) and mostly non-functional sperm (shown in white). In (b) the mature oocyte is ovulated into the spermatheca, which is the site of fertilization. Each oocyte will potentially contact all of the sperm, both functional and non-functional that are stored in the spermatheca simultaneously. After a short interval, the oocyte is pushed into the uterus (c) and any sperm that have also been displaced from the spermatheca into the uterus will make their way back into the spermatheca and await the next ovulation event. Although some of these displaced sperm might contact any unfertilized oocytes in the uterus, no fertilization occurs outside the spermatheca. In Nomarski images, examples of sperm are indicated by black arrowheads.