Sections of mitochondria in Paramecium and Euplotes present a consistent pattern. The mitochondrion in these cells can be conceived of as a twisted mass of closely compacted tubules. Two general kinds of substances can be recognized: the electron-dense that borders the lumen of the tubule, and the less dense that forms the continuum. In sections of mitochondria in rat kidney and snail oviduct, tubular internal organization can be recognized. In the same organs, mitochondria with lamellar internal structure can be demonstrated. The thesis is developed that the mitochondrion is a structure capable of differentiation and change, and that developmental continuity among the different kinds may exist. Mitochondria that appear to be different may be quite similar basically; mitochondria that appear to be similar in structure may be different in other ways. The tubule is proposed as the most basic of the presently recognized mitochondrial structures.
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.