Every female fruit fly is equipped with a pair of ovaries which remain connected by a stalk (shown in green). The Drosophila ovary is a marvel of biological engineering: each ovary is made up of approximately 16 ovarioles which are set up in assembly line fashion. Stem cells residing at the tip of each ovariole divide to give rise to a single cell that subsequently undergoes four additional divisions. After it has completed these divisions a 16-celled 'cyst' is formed, with all 16 cells remaining connected. Only one cell will become the oocyte, or egg, while the remaining 15 are 'nurse' cells- they support and provide the developing oocyte with the nutrients it needs to grow as well as instructions for making certain proteins. Eventually the nurse cell contents fuse with the oocyte and a mature egg is formed. Each cyst is separated from other developing cysts by a single layer of cells and a short stalk, forming structures known as egg chambers.This entire process is repeated throughout the life of the female, and so each ovariole is really a string of egg chambers at different stages of development- the youngest cysts are closest to the stem cells, while the oldest cysts and eggs are clustered at the very end of the assembly line.
That's the simple explanation, at least, but throughout egg formation many processes are occuring simultaneously, and there is a lot of cross-talk between all of the different populations of cells within the ovary and between cysts. It is only through a massive concerted effort that each egg is formed with all of the necessary instructions laid out in preparation for creating a fully formed fruit fly larva after fertilization.