56 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



arranged in rows under the cell surface. Under certain conditions 

 these may discharge their contents as sticky, ductile filaments. It 

 is generally agreed that this material is a mucoid or gelatinous 

 substance normally contributing to the formation of amorphous 

 cyst or other, possibly protective, external coatings. Presumably 

 the so-called protrichocysts of some ciliates are analogous 

 structures. The latter appear as spheroidal or pear-shaped bodies 

 of low density in thin sections of tetrahymenid ciliates (Pitelka, 

 1961a; Grassland Mugard, 1961). Following discharge, regenerat- 

 ing mucigenic bodies in Ophryoglena (Grasse and Mugard, 1961) 

 are visible as long rods with dense contents ; their density decreases 

 as they mature. In a brief, unillustrated abstract, Dragesco (1952d) 

 states that discharged filaments of mucigenic bodies in several 

 flagellates and ciliates are very variable in size and show no 

 definable structure in electron micrographs. 



Structures that in the light microscope appear to be fibers occur 

 in nearly limitless variety of form and arrangement in protozoa, 

 particularly in ciliates and flagellates. These cannot be described 

 without detailing the morphology of the species concerned; hence 

 they will be dealt with in the next chapters and summarized in the 

 concluding section of this book. It may be noted, however, that 

 the highly ordered structure of the discharged trichocyst of the 

 Paramecium type is not unique. Other protozoan fibers, although 

 constructed of quite different units, likewise display paracrystalline 

 patterns of impressive precision. 



Nuclei 



In general, electron microscopy of nuclear structure has been 

 disappointing. The problem, at least in protozoan studies, is not 

 that nothing of interest has been seen, but that a number of 

 different things have been seen, difficult to reconcile in terms of a 

 unifying story of nuclear structure and behavior. This of course 

 has always been true in protozoan nuclear cytology, but in recent 

 years critical light-microscope investigations have permitted most 

 of the divergent patterns and processes to be interpreted in 

 conventional karyological terms (see Grasse, 1952; Grell, 1956; 

 Nanney and Rudzinska, 1960); protozoa may be highly uncon- 

 ventional in some details of their mitotic and meiotic cycles, but 

 they accomplish the same results, with the same basic materials, 



