162 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



of a typical fibrillar centriole embedded in the dense hemispherical 

 complex atop the rostral tube of Trichonympha is of outstanding 

 interest. Thanks to Cleveland's comprehensive light-microscope 

 studies (1957, 1960, and many previous papers cited therein) it 

 would seem that metamonad zooflagellates provide a choice 

 material for the electron-microscope study of centriolar morpho- 

 logy and behavior, and it had been disappointing to find no 

 structure in the presumed centrosome regions of Trichonympha^ 

 Foaina, and Pyrsonympha that could be homologized with the 

 centrioles of other cells. In all of these instances, the centrosome 

 area is electron-dense; Grimstone's observation raises the 

 possibility that fibrillar elements may be detected in them also if 

 improved staining techniques are applied. 



A similar dense zone was seen by Rouiller and Faure-Fremiet 

 (1958a) near the kinetosomes of the chrysomonad Chromulina 

 psammobia; this is the only instance among the phytoflagellates 

 for which a possible centriole distinct from a kinetosome has 

 been reported from electron-microscope studies (although others 

 are described in the light-microscope literature), and the question 

 needs further study. In general — and speaking from a void, 

 since no high-resolution studies of mitotic phenomena in flagel- 

 lates have been published — it seems that, as long as the number 

 of flagella remains small, the cluster of kinetosomes may serve all 

 morphogenetic, kinetic, and mitotic functions required of them. 

 As the flagellar complement increases, and as the number of 

 associated organelles multiplies, the problems involved in estab- 

 lishing cell polarity and controlling the processes of division 

 become more complex, and a division of labor in the organiza- 

 tional centers may be necessary. In the higher zooflagellates, a 

 pair of " master " kinetosomes is segregated, which retain all 

 the multi-potent generative and organizational capacities, while 

 somatic kinetosomes are limited to the production and control of 

 elements in their immediate environment. An electron-micro- 

 scope tour through the zooflagellates, guided by the facts estab- 

 lished in Cleveland's works, should permit us to trace the 

 anatomical and functional segregation of kinetosomes and 

 centrioles in this series. 



In the opalinids and ciliates, polymerization of flagellar units 

 poses the same organizational problems, with additional special 



