18 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



rather curious structure occurs (Pitelka, 1961b). Internal mem- 

 branes have the form of randomly oriented flat discs (Figs. 1 and 5, 

 PL I); only rarely do they appear to be in continuity with the 

 inner limiting membrane. In Trichonympha (Pitelka and Schooley, 

 1958) and in Opalina (Noirot-Timothee, 1959), both intestinal 

 symbionts, bodies with a much finer tubular internal structure 

 than usual are tentatively identified as mitochondria. 



Pappas and Brandt (1959) found in the giant ameba, Pelomyxa 

 carolinensis , at the time of nuclear division, some mitochondria 

 with a conspicuous pattern of organization. Its main theme 

 consisted of microtubules with regular zigzag profiles. Where 

 parallel tubules were in phase, a strikingly constant space, about 

 25 m/x wide, separated them; the space within the tubules was 

 wider and more variable, 40 to 70 m^. Elsewhere, adjacent 

 out-of-phase zigzags apparently fused at their peaks to produce 

 regularly fenestrated plates, and three-dimensional interdigitation 

 of neighboring zigs and zags produced complicated but orderly 

 geometric patterns. Somewhat similar configurations have been 

 illustrated in the mitochondria of developing spermatids (Andre, 

 1959). 



No explanation for the occurrence of these remarkable patterns 

 is available. Nor is any consistent functional significance ascribable 

 to the difference between cristal and microtubular form in the 

 internal membranes. Osmotic properties of microtubular mito- 

 chondria isolated from Acanthamoeba (Klein and Neff, 1960) 

 proved similar to those of mammalian mitochondria. Vickerman 

 (1960) found that Acanthamoeba mitochondria contained dense 

 inclusions within the microtubule lumina, and that these were 

 significantly more numerous in cysts than in active cells. Stewart 

 and Stewart (1961) similarly have reported abundant dense 

 inclusions in the mitochondria of the slime mold Physarum in the 

 dormant, sclerotized phase. But the significance of the inclusions 

 is unknown. 



Mitochondria are known to be the principal site of oxidative 

 phosphorylations in aerobic cells (Lehninger, 1959; Green and 

 Hatefi, 1961). Many enzymes of the respiratory chain are bound on 

 or within the mitchondrial membranes, apparently in precisely 

 linked assemblies that are repeated thousands of times within a 



