46 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



subfibrils of adjacent fibrils, as well as a central cartwheel structure, 

 appear distinctly in micrographs of snail sperm centrioles by Gall 

 (1961). Animal cell centrioles typically are present at least in 

 duplicate during interphase ; each is a short, open, fibrous cylinder 

 and the two generally are oriented at about right angles to each 

 other. Sotelo and Trujillo-Cenoz (1958a, 1958b) have studied the 

 origin of flagella from preexisting centrioles in developing 

 spermatids of arthropods and vertebrates and in embryonic chick 

 neural epithelium. Both in the spermatids and in the epithelial 

 cells the centrioles migrate to the cell periphery, where one of 

 them becomes oriented perpendicular to the surface and functions 

 as a kinetosome. The peripheral flagellar fibrils either grow out 

 from the kinetosomal fibrils or are organized from vesicles in the 

 flagellar bud and become continuous with the kinetosomal fibrils. 

 Central fibrils appear at the same time. Following establishment 

 of contact or at least contiguity between the distal end of the 

 flagellum-forming centriole and the cell membrane, the kineto- 

 some withdraws into the cell, carrying with it the cell membrane 

 wrapped around the young 9+2-patterned flagellum and lining 

 the pit that it occupies. Subsequently, the pit may disappear as 

 the kinetosome returns to the cell periphery. 



No similar observations on the sequence of development of 

 protozoan cilia are yet available, nor do we know how the fibrous 

 structure of kinetosomes is reproduced when these organelles 

 duplicate. Roth (1960b), studying division stages in the hypotrich 

 ciliate Stylonychia, found groups of ciliary fibrils embedded in the 

 cytoplasm, not immediately adjacent to existing intact cilia, at a 

 time when new ciliary organelles are expected to be developing. 

 Some of these appear to have central fibers, some do not, and 

 some seem to be incomplete cylinders. The implication that 

 kinetosomes are built up gradually from morphologically 

 undifferentiated precursors is suggested, but, as Roth recognized, 

 further study would be necessary to substantiate this. This is 

 especially so since old ciliary organelles in the hypotrich ciliates 

 often dedifferentiate while the new ones form or soon thereafter. 

 Dedifferentiating flagella have been seen in the multiflagellated, 

 syncytial zoospores of the alga Vaucheria, by Greenwood (1959). 

 Upon settling on a substrate these organisms withdraw their 

 flagella into the protoplasm. Sections show clearly that the 11 



