48 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



cell surface, in a cell where cilia, according to all previous 

 knowledge, had no business to be at all. 



The whole question of genetic continuity of centrioles and of 

 kinetosomes is one of long standing (see Lwoff, 1950; Gall, 1961). 

 The equivocal nature of morphological data, even at the ultra- 

 microscopic level, is not surprising. If new kinetosomes are 

 organized independent of old ones, then there may be nothing to 

 be seen until the new kinetosome is there, no way of recognizing 

 that it is about to be there. On the other hand, if an old kineto- 

 some is required either as a physical parent or as a template for the 

 construction of a new one nearby, morphological problems are 

 not much less formidable. The sample available to electron 

 microscopy is so minute, in both time and space, that a rapid 

 process occurring in a very small area is excessively difficult to 

 catch. In protozoa, kinetosome replication commonly precedes 

 any other sign of morphogenetic activity, so that selection of 

 favorable cells for study is not easy. Probably the best bets are 

 cells regenerating their surface structure following surgical 

 mutilation, or species with such prolonged but rigidly scheduled 

 differentiation programs that the time of replication of specific, 

 identifiable kinetosome groups can be accurately predicted. In 

 any case, long sequences of sections, covering enough area to 

 permit accurate topographic identification, will be necessary. 

 However, in view of the intense investigation under way in 

 laboratories all over the world, employing morphological and 

 biochemical methods, early resolution of some of these problems 

 is to be expected. 



The doubled condition of centrioles in resting animal cells 

 reminds one of the fact that the great majority, at least, of 

 flagellated protozoa bears two or more flagella. Usually the two 

 kinetosomes of a biflagellate are close together but oriented at 

 an angle (not necessarily 90°) to one another, or if parallel they 

 show opposite polarity. As will be seen in the succeeding chapters, 

 several presumed uniflagellates have turned out to have second 

 kinetosomes which either are blind or terminate in short flagella 

 not detectable with the light microscope. The weight of present 

 evidence would indicate that in a few very simple cells a true 

 uniflagellate condition obtains; that is, that kinetosomes can 

 exist singly. Such appears to be the case in three small green 



