Nebenkern
The nebenkern is a mitochondrial formation in the sperm of some insects such as Drosophila. After the completion of meiosis, spermatid mitochondria wrap around each other to form a spherical aggregate, adjacent to the nucleus. The nebenkern proceeds to elongate into a double-stranded helical structure. During flagellum elongation the nebenkern unfolds and the two derivatives elongate down the bundle of microtubules that constitute the axoneme core of the flagellum.
