66 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



1955) or a tetrahymenid (Fig. 26, PL VII) average about 100 to 

 200 m/x in diameter, and they are believed to represent the 

 Feulgen-positive material of the macronucleus. Larger, more 

 discrete dense bodies are probably nucleoli. 



The ciliate macronucleus is hyperpolyploid, a small fragment 

 sufficing to regenerate a genetically normal whole (see Sonneborn, 

 1949; Faure-Fremiet, 1953; Grell, 1956). As is well known, it is 

 derived from division products of the micronuclear synkaryon 

 following sexual reproduction. Cytological and cytochemical 

 methods show a many-fold increase (up to 200 x ) in DNA content 

 in the macronuclear anlage, associated with repeated endomitoses ; 

 ultimately the macronucleus is totally responsible for nuclear- 

 controlled phenotypic characters of the cell. 



The indefinite survival of amicronucleate races of ciliates proves 

 that in them a periodic renewal of the macronucleus is not essential. 

 Many ciliate macronuclei, however, in addition to being replaced 

 following sexual processes, are periodically reorganized in some 

 manner, as evidenced by a condensation and change in staining 

 properties accompanying amitotic vegetative division, or by an 

 interphase phenomenon of progression of visible reorganization 

 bands through elongate macronuclei. The significance of these 

 processes relative to the necessary doubling of the macronucleus 

 has until recently been unknown. Happily, a critical electron 

 microscope examination of the reorganization phenomenon in the 

 hypotrich, Euplotes eurjstomns, by Faure-Fremiet, Rouiller, and 

 Gauchery (1957) was followed by a cytochemical and tracer study 

 of the same species by Gall (1959 — Gall, however, was unaware 

 of the earlier work). 



In E. eurystomus, the very long, slender macronucleus in early 

 interphase is filled with small, basophilic, Feulgen-positive 

 granules. At a time about halfway between fissions, clear, weakly 

 staining, transverse bands appear at both ends and progress 

 slowly toward the center, where they disappear a few hours before 

 the next division, leaving in their wake coarser granules showing 

 more intense basophilia than before. In electron micrographs the 

 reorganization band is easily recognized ; at its medial (advancing) 

 face, dense 1 \l bodies corresponding to those seen in the light 

 microscope are abruptly cut off, apparently by a process of 

 disaggregation, and are replaced by a fine reticulum in the 



