1. Observations on the morphological changes of mitochondria preparatory to the formation of the nebenkern, as well as changes within the nebenkern, are reported.
2. Mitochondria enlarge and divide during the meiotic divisions.
3. The mitochondria of the spermatid elongate, become filamentous, form a crescent, and partially encircle the nucleus.
4. Nodes which develop on either end of the crescent become entwined as they move toward each other.
5. The two nodes coalesce to form a filamentous or early type nebenkern which is described by others as chromophilic.
6. Internal rearrangement and partial dissolution of the filaments result in the development of the late or chromophobic nebenkern which separates into two distinct bodies.
7. The nebenkern moieties send out processes toward the centrosome, and after making contact, elongate and occupy part of the space between the tail filaments and sheath of the spermatozoon.