A tiny little roundworm
Caenorhabditis elegans is a one millimeter-long soil roundworm, as well as an insightful model organism for research in molecular and developmental biology, because it is simple, easy to grow and can be frozen. C. elegans has two sexes: a self-fertilizing hermaphrodite and a male. Hermaphrodites make both sperm and eggs. This picture of a hermaphrodite shows its extruded gonad that contains many germ cells (blue), including sperm (red) and eggs (green).
Courtesy Kyung Won Kim and Judith Kimble, UW-Madison Biochemistry department and the Howard Hughes Medical Institute.