An ovum and sperm
A sperm was a type of cell essential to Human procreation. Once a sperm fertilized an ovum, the development of tissue could begin.
The Doctor's lesson plan for Seven of Nine's instruction in interpersonal relationships, "Love Amid the Stars: A Romantic Bestiary", included information on the ovum as part of "Lesson One: First Contact". While showing her a diagram of the Human procreation process, he described the image by saying, "Here we see how fortress ovum is besieged by countless little warriors (the sperm).". When Seven told him she was familiar with the biology of sexuality, he skipped ahead to "Lesson two: Encounter in a Public Place." (VOY: "Someone to Watch Over Me")