78 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



dark, dense, strongly dehydrated spore of the slime mold, 

 Didymium. But in others, including a streaming channel of the 

 slime mold plasmodium, textural differences are observed within 

 very small regions of the same cell. In many instances, lamellar 

 or vesicular elements and large cytoplasmic inclusions are absent 

 from the zones assumed to be in a gel state. 



Lehmann and various colleagues (see especially Lehmann, 

 1958a) have demonstrated repeatedly that the application of 

 fixatives other than the customary, slightly alkaline, osmium 

 tetroxide to Amoeba proteus can result in a distinctly reticular or 

 fibrous structure in or just beneath the plasmalemma and around 

 the wall of the contractile vacuole. They have not yet succeeded 

 in compounding a fixative that preserves both this structure and 

 that of other cytoplasmic organelles. Much of their evidence has 

 come from isolated cell fragments treated with various acidic 

 mixtures, and many workers consider that more artifacts may be 

 introduced than avoided by these techniques. 



That local variations in cytoplasmic texture can be detected at 

 high resolution in well-fixed cells is conclusively demonstrated 

 by Wohlfarth-Bottermann's work; this much is encouraging. 

 The extremes of these variations are less spectacularly different 

 than we might have hoped, and this means that correlating them 

 with known physical states or activities will be no easy matter. 

 They are even less conspicuous in the giant amebae that have been 

 most frequently used for analyses of movements than in the 

 comparatively neglected smaller forms. Furthermore, the exis- 

 tence of tiny pseudopodia and surface irregularities unsuspected 

 by light microscopists suggests that underlying processes must be 

 examined on a more minute scale than most workers have 

 attempted. 



Except in inclusion-free regions of the ectoplasm, the cytoplasm 

 of Hyalodisais , and other small amebae, contains membranous 

 fine tubules and vesicles of varying sizes and shapes (Fig. 29, 

 PI. VIII). In Amoeba and Pelomjxa the cytoplasm is virtually 

 filled with such vesicles. Variations in the appearance of the 

 vesicle membranes and their possible significance have been 

 discussed in the section on pinocytosis and phagocytosis in the 

 preceding chapter. Most of the cytoplasmic membranes in the 

 giant amebae are smooth, but occasional ones are seen bearing 



