222 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



are very common in ciliated cells of metazoa; they frequently 

 originate at the base of a tapering, closed kinetosome and usually 

 pass medially, resembling the rhizoplasts of flagellates rather than 

 the kinetodesma of ciliates. 



The only fibrils that appear unquestionably to be direct out- 

 growths of kinetosomal fibrils are those of the flagellum itself, 

 sometimes converted into something else as in the stalks of 

 peritrichs. 



Among the tubular and filamentous fibrils, a large proportion 

 is known to be, or suspected of being, connected with kineto- 

 somes, but there remain some for which such a suspicion seems 

 tenuous, although it cannot be eliminated. Conceivably a 

 centriole could be so located as to participate in the organization 

 of pellicular fibrils in the sporozoans, for example, and the 

 pellicular fibrils of Opalina might originate at kinetosomes of the 

 falx area, which has not been determinedly studied. But the 

 necessity or permanence of this relationship should not be 

 exaggerated. In Euglena, for example, where a relatively small 

 number of fibrils takes off from the kinetosomes to join the 

 pellicular system in the reservoir, the system over the body 

 surface embraces a large number of parallel fibrils, and some of 

 the surface striae do not enter the reservoir at all. In several 

 zooflagellates, the axostyle seems not to connect directly with the 

 centriole/kinetosome complex but rather to be linked to it by 

 fibrils of a different kind. New axostyles do, however, grow out 

 from the centrosomal region. In Tetrahymena, the longitudinal fibril 

 bands clearly have no direct connection with kinetosomes. In the 

 ophryoscolecids, filamentous layers occur in profusion all over a 

 body that has a relatively small ciliated zone. An aspect of the 

 problem of fibril morphogenesis might be approached by studying 

 Tetramitiis, in which an ameboid cell may transform into a 

 flagellated one (a process probably involving the appearance of 

 pellicular fibrils as well as of flagella) within less than an hour. 



The multiplicity of kinds (appearances !) of fibrils arising in the 

 kinetosome region poses many unanswerable questions. A 

 tetrahymenid somatic kinetosome somehow "knows" that it has 

 to organize one kind of fibril at its right anterior margin and 

 another kind at its right posterior margin and left side; perhaps 

 these events occur at different times when different precursors are 



